Two pipes P and Q can separately fill cistern in 6 hours and 8 hours respectively, while a third pipe R can empty it in 12 hours. If all the three pipes are opened together, then the time taken to fill the cistern is

A

4 hrs. 36 min.

B

4 hrs. 48 min.

C

4 hrs. 12 min.

D

4 hrs. 40 min.