GMATPrep DS Sample Question

The answer explanation for this problem just does not seem right to me. I chose B statement 2 alone, but it says E neither are sufficient. Please explain.

In a refinery, the capacity of oil tank A is 70% of the capacity of oil tank B. How many more gallons of oil are in tank A than in tank B?

(1) Tank A is 90% full; tank B is 50% full.
(2) When full, tank A contains 50,000 gallons of oil

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.

Answer Explanation

Since you do not know the number of gallons in either tank A or tank B, (1) alone is not sufficient. From (2) alone, you can determine the capacities of tanks A and B, but you do not know whether the tanks are full. Using (1) and (2) together, you can determine the number of gallons in A and B; therefore, BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.