There are two gambles to consider. The original by the king, and the second by prisoner A. The first gamble had a 1 in 3 odd for winning. This does not change because prisoner A knows the outcome for C. The second gamble is whether A choose to play between cells A and B, which would be a 1 in 2 chance. However, this still falls under the first gamble which is 1 in 3. Multiplying these two chances together equals a 1 in 6 chance, versus a 1 in 3 chance under the king only gamble. Staying in cell A and not playing is the better choice.