Answer C

Hi Mayatassi, We are actually told that the results of the game are quite predictable, most people reject the offer when it is insultingly low. While this is hard to explain, this is not the same thing as it being hard to predict.

Why is B wrong?

I think b just goes a little too far. While it does seem to contradict the idea that people always act in their rational self interest, we aren't told that this assumption is basic or foundational, and one piece of disconfirming evidence doesn't necessarily overturn an assumption.