Which offer is more irresistable?

Copywriter extraordinaire Bob Bly posted this interesting quiz for marketers on his blog recently. It's quick, but not easy. Ready?

Here are three different offers:

(A) Half price.
(B) Buy one, get one free.
(C) 50% off.

One of them pulled 40% more replies than the other two.

Which do you think was the winner? And why?

Post your guess below by clicking "Comment". Then go look at the correct answer.