John-Paul Volunteers to Help His Science Teacher with a Demonstration

John-Paul volunteers to help his science teacher with a demonstration.The teacher hands John-Paul a card with a dot on the left-hand side and an X on the right-hand side.Following the teacher's instructions,John-Paul closes his right eye,focuses on the X,and slowly moves the card toward his face.The dot seems to disappear magically,though of course when he opens both eyes it is still there.The teacher explains that this is caused by a blind spot.What causes blind spots?

A) Blind spots are a dominant genetic trait that serves no real purpose;some people don't have them.
B) The ciliary muscles are attached in two places to the retina;they block some of the eye's photoreceptors.
C) Blind spots are like burned out pixels in a computer screen;they occur when ganglion cells "wear out."
D) The optic nerve exits through the back of the eye,and there are no photoreceptors in that spot.