[pzhon]

B is right if you assume that you have two cards that are not the ace of spades. It's overly complicated in this case, but that technique is often useful.

A is wrong. The probability that the first card on the flop is the ace of spades is indeed 1/50. The probability that the second card on the flop is the ace of spades is also 1/50, not 1/49. If you are given the extra information that the first card is not the ace of spades, then the conditional probability (that is, using the information) is 1/49. You need to add up the actual probabilities, not the conditional probabilities from different contexts.

The simplest correction to A is

A. 1/50 + 1/50 + 1/50 = 6%.