Work Problem....a little help please

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 the small pump to fill the tank working alone? (A) 8 (B) 9 (C) 12 (D) 15 (E) 18 OA is...