The trick to answering this question is to realize that the question is about average sales.

So once you have the average monthly sales for the entire several year period, you can calculate the average sales for any number of years within the period as well.

For example, if the average monthly sales were 100. Then the average sales for 12 months, in other words one year, would be 1200.

Statement 1:

This gives us no quantity for sales that could be used to calculate average sales.

Insufficient.

Statement 2:

This provides the average monthly sales, which can then be used to calculate the average biannual, in other words two year, in other words twenty-four month, sales.

Sufficient.

The correct answer is B.

Cute question.

This is one of those questions that might have a much higher difficulty rating than you think; in our experience, these "simple" questions with even simpler traps tend to catch almost everyone offguard, and can have the lowest correct response % of anything on the test, even lower than some questions that are much more conceptually difficult. (Those tend to have response rates between 20% and 50%, depending on how easy it is to eliminate some of the answers before guessing.)