A pipe can fill a cistern in 12 minutes and another fill it in 15 minutes, but a third pipe can empty it in 6 minutes. The first two are kept open for 6 minutes in the beginning and then the third pipe is also opened, in what time is the cistern emptied?

• A. 54 min
• B. 56 min
• C. 58 min
• D. 60 min