Two inlet pipes can separately fill a cistern completely in 8 hours and 10 hours respectively. They are operated for 2 hours, after which the second pipe is closed, and an outlet pipe which can drain out water from the full cistern in 20 hours is opened. How much time will it take to fill the cistern completely from the instant of opening the outlet pipe?

A

7 hours

B

8 hours

C

7 hours 20 minutes

D

8 hours 40 minutes