Answer: C

It may be tempting to come up with a lot of variables, one each for each month's revenue for each company. However, focus on the differences. In the first month, the difference was +40 in favor of A. (Note that we can drop the thousands since every number in the question is in terms of thousands.) In the second, the difference was +8 in favor of A. The average was +2 in favor of A.

With these numbers, use the average formula to find the third month (T):

(40 + 8 + T)/3 = 2
48 + T = 6
T = -42

Since positive numbers indicate a difference in favor of A, negative numbers are in favor of B. -42 represents a $42,000 advantage in favor of Store B. Choice (C) is correct.