For the Next Group of Questions, Consider the Following Argument

For the next group of questions, consider the following argument:
(1) Marla's boat is at the dock and (2) every one of the boats at the dock has loaded either three or four passengers.(3) But I'm sure that Marla's boat has only loaded three passengers, because (4) she would already have raised her sails if she had all four of her passengers.
-What is the conclusion of the argument?

A) Marla's boat is at the dock.
B) There are other boats at the dock.
C) Every one of the boats at the dock has loaded either three or four passengers.
D) Marla's boat has only loaded three passengers.
E) Marla would already have raised her sails if she had all four of her passengers.