**Source: GMAT Paper Tests**

Working alone, a small pump takes twice as long as a large pump takes to fill an empty tank. Working together at their respective constant rates, the pumps can fill the tank in 6 hours. How many hours would it take for the small pump to fill the tank working alone?

A. 8

B. 9

C. 12

D. 15

E. 18

The OA is E