A cistern has two pipes attached to it, one to supply and one to draw off. If both the pipes are opened together, the cistern is filled in 9 hours but if the waste pipe is opened one hour after the supply pipe, the cistern is filled in 7 hours. The time that will be taken by the supply pipe to fill the empty cistern is:

A

2 hrs. 47 minutes

B

3 hrs. 30 minutes

C

3 hrs. 32 minutes

D

3 hrs.