From Twitter:

A. You think Earth is flat?
1. B. Yes. __ it isn't.
2. A. I can show you the __.
3. B. What's your __?
A. Ever seen a lunar eclipse?
B. Sure.
A. Earth's shadow is curved on the moon.
B. That's the moon's curve!
A. Ug. So--fly to 13,624 ft, or higher!
B. Oh.

a. proof
b. evidence
c. prove

Answers and Explanation:
1. c., prove (a verb)
2. a., proof or b., evidence (both nouns)
3. a., proof or b., evidence (both nouns)