Which is the best example of a reason?

A. First of all, if someone says your dream is impossible, they’re wrong.

B. You can’t prove that something is impossible—all you know is that [it] hasn’t been done yet.

C. People said no one could scale Mount Everest, but in 1953, Sir Edmund Hilary and Tenzing Norgay gazed down upon the world from its summit.

D. People said that man could not fly, but in 1903, Orville and Wilbur Wright made the first sustained powered flight in a heavier-then-air machine.