Passage
In 1482, a thirty-year-old Leonardo da Vinci left his art studio in Florence and traveled to Milan to seek employment at the court of Duke Ludovico Sforza. A drafted letter of application to the duke detailed Leonardo's many useful skills, the majority of which concerned military developments such as "methods for destroying any fortress" and making "unassailable armored chariots." Peacetime talents such as "guiding water from one place to another" or producing "sculpture in marble, bronze and clay" were mentioned only at the end of the letter. Nevertheless, one of the most intriguing items is the entry stating, "I have designed extremely light and strong bridges, adapted to be easily carried": in other words, portable bridges.Modern scholars question whether most of the devices mentioned in Leonardo's letter could really have been constructed. For instance, Krull describes the fact that Leonardo never sent the letter as "a good thing, as it was mostly bluff." Isaacson, though a bit more charitable, similarly declares that Leonardo's "boasts were aspirational." Regarding the bridge, however, there is no doubt that the design is workable.According to at least one account, Leonardo really did make such a bridge for Sforza. More significantly, examples of the bridge have been built and used in contemporary times. A sketch in one of Leonardo's notebooks illustrates the bridge's construction, which includes different sets of wooden beams cut to specific lengths. Requiring no nails, ropes, or other fasteners, nor any tools, the bridge can be assembled by a mere handful of people, and even some children could potentially build a small version or model.When the bridge is constructed correctly, each component helps to hold all the other components in place. The original sketch calls for the beams to be notched so that the pieces of the bridge lock together. However, in some cases this feature can be omitted, provided the wood is sufficiently rough. This version of the bridge then stands and bears weight due to friction, gravity, and the physical integrity of the pieces. In this way, the structure depends entirely on the interplay of physical forces upon its parts.The design is modular and repeatable, meaning that a longer bridge can be made by simply adding another section, with the supports interlocked accordingly. Given enough materials, time, and manpower, such a bridge could in principle be made to quite a great length. However, in longer bridges the notches would in fact be needed, because some of the beams would be at steeper angles.Despite the relative ease of constructing the bridge, there is one challenge: building it requires access to both sides of the region to be crossed, as well as the area in between. Such a consideration might seem obvious in building any bridge, but it can still prove problematic. If a bridge is meant to help cross a body of water, not all members of the group attempting to cross will receive its benefit, as some would have to stand in the water during the bridge's construction. Of course, this assumes that it is safe to enter the water in the first place. Crossing a chasm or a deep pit would pose even greater difficulties, as the bridge's prospective builders would, like the ancient Greek mathematician Archimedes, lack a place to stand. Leonardo may have confronted that problem as well and considered how to make a bridge that was not only portable and freestanding, but constructible in sequence from one end to the other. Although no evidence exists of such a design in the known pages of his notebooks, many pages have been lost to history and might one day be rediscovered.
-Based on passage information, the author's primary reason for believing that the portable bridge is a workable design is that:

Question

Passage
In 1482, a thirty-year-old Leonardo da Vinci left his art studio in Florence and traveled to Milan to seek employment at the court of Duke Ludovico Sforza.  A drafted letter of application to the duke detailed Leonardo's many useful skills, the majority of which concerned military developments such as "methods for destroying any fortress" and making "unassailable armored chariots."  Peacetime talents such as "guiding water from one place to another" or producing "sculpture in marble, bronze and clay" were mentioned only at the end of the letter.  Nevertheless, one of the most intriguing items is the entry stating, "I have designed extremely light and strong bridges, adapted to be easily carried": in other words, portable bridges.Modern scholars question whether most of the devices mentioned in Leonardo's letter could really have been constructed.  For instance, Krull describes the fact that Leonardo never sent the letter as "a good thing, as it was mostly bluff."  Isaacson, though a bit more charitable, similarly declares that Leonardo's "boasts were aspirational."  Regarding the bridge, however, there is no doubt that the design is workable.According to at least one account, Leonardo really did make such a bridge for Sforza.  More significantly, examples of the bridge have been built and used in contemporary times.  A sketch in one of Leonardo's notebooks illustrates the bridge's construction, which includes different sets of wooden beams cut to specific lengths.  Requiring no nails, ropes, or other fasteners, nor any tools, the bridge can be assembled by a mere handful of people, and even some children could potentially build a small version or model.When the bridge is constructed correctly, each component helps to hold all the other components in place.  The original sketch calls for the beams to be notched so that the pieces of the bridge lock together.  However, in some cases this feature can be omitted, provided the wood is sufficiently rough.  This version of the bridge then stands and bears weight due to friction, gravity, and the physical integrity of the pieces.  In this way, the structure depends entirely on the interplay of physical forces upon its parts.The design is modular and repeatable, meaning that a longer bridge can be made by simply adding another section, with the supports interlocked accordingly.  Given enough materials, time, and manpower, such a bridge could in principle be made to quite a great length.  However, in longer bridges the notches would in fact be needed, because some of the beams would be at steeper angles.Despite the relative ease of constructing the bridge, there is one challenge: building it requires access to both sides of the region to be crossed, as well as the area in between.  Such a consideration might seem obvious in building any bridge, but it can still prove problematic.  If a bridge is meant to help cross a body of water, not all members of the group attempting to cross will receive its benefit, as some would have to stand in the water during the bridge's construction.  Of course, this assumes that it is safe to enter the water in the first place.  Crossing a chasm or a deep pit would pose even greater difficulties, as the bridge's prospective builders would, like the ancient Greek mathematician Archimedes, lack a place to stand.  Leonardo may have confronted that problem as well and considered how to make a bridge that was not only portable and freestanding, but constructible in sequence from one end to the other.  Although no evidence exists of such a design in the known pages of his notebooks, many pages have been lost to history and might one day be rediscovered.
-Based on passage information, the author's primary reason for believing that the portable bridge is a workable design is that:

Quizplus · Accepted Answer

The author states that "there is no doubt that the [bridge's] design is workable" (Paragraph 2) and goes on to explain: "According to at least one account, Leonardo really did make such a bridge for Sforza. More significantly, examples of the bridge have been built and used in contemporary times" (Paragraph 3). This statement indicates that the author's primary reason for believing that the portable bridge is a workable design is that it has served as the model design for some present-day bridges. By contrast, none of the following is the primary reason why the author believes that the portable bridge is a workable design: Leonardo is said to have really built such a bridge on at least one occasion (Choice A). The author indicates that "[a]ccording to at least one account, Leonardo really did make such a bridge for Sforza." However, the author then states that "[m]ore significantly, examples of the bridge have been built and used in contemporary times," asserting the primary reason for his or her belief that the bridge is a workable design (Paragraph 3). it was a realistic and practical idea, unlike Leonardo's more fanciful military designs (Choice C). Although Leonardo's military designs may have been more fanciful than the bridge, the author's primary reason for believing that the bridge was workable is the examples of it "built and used in contemporary times" (Paragraph 3), rather than a comparison with Leonardo's other, "aspirational" ideas (Paragraph 2). its construction is based on the repetition of a modular structure (Choice D). Paragraph 5 states that the bridge's "design is modular and repeatable." However, the author's primary reason for believing the design to be workable is that "examples of the bridge have been built and used in contemporary times" (Paragraph 3).

Passage In 1482, a Thirty-Year-Old Leonardo Da Vinci Left His Art