OT: Blind Guess on a Test!

There was a situation that arose during on of my finals earlier this month where I was clueless as to what the right answer was. I decided that a blind guess was in order but wait the answers confused me. Common knowledge would dictate they are all equal in their percentage to be right but it seems they wern't.

Here is how it was laid out.

A. Answer 1
B. Answer 2
C. All of the Above (Answer 1 and 2)
D. None of the Above (Not Answer 1 and 2)

It seems the dependency of C and D alters the percentages but after working it out I came to the conclusion they were still equal. Can someone explain this mysterious situation to me?