Three pipes A, B and C can fill a tank in 7 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 9 hours. How many hours will take C alone to fill the tank?	Choose the correct choice from the following answers.
A) 42/5 B) 47/4 C) 56/3 D) 63/4 E) 70/8
The rate of A+B+C is 1/7 of the tank per hour.   After 2 hours, the tank is 2/7 full.   The rate of A+B is 5/7 * 1/9 = 5/63 of a tank per hour.   The rate of C is 1/7 - 5/63 = 4/63   C can fill the tank in 63/4 hours.   The answer is 63/4.
<result>D</result>
Three pipes A, B and C can fill a tank in 7 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 9 hours. How many hours will take C alone to fill the tank?