Hint:
Let N be the number of
satisfying assignments to the CIRCUIT-SAT instance.
First show how to design algorithm B_k that decides membership
in CIRCUIT-SAT using oracle access to A when it is guaranteed that
either N=0 or N is betwee 2^{k-1} and
2^k. (You might want to recall the result you proved
in the first part of the previous
homework problem).