Earth, Flat, Hook, Sharp

Meredith's anthropology professor posed the following question:

Of the following choices, which does not belong: Earth, Flat, Hook, Sharp. Why?

I have no idea what the correct answer is or even if there is one. I will explain my own theory in a follow-up post. What's your answer?

Edited to Add: Formulate your own theory first before peeking at the comments!