Four pipes A, B, C and D can fill a tank with water in 15, 20, 30 and 60 hours respectively. Pipe A is opened at 4 a.m., B at 5 a.m., C at 6 a.m. and D at 7 a.m. When is the tank filled up completely?

A

9:30 a.m.

B

10:00 a.m.

C

10:30 a.m.

D

11:00 a.m.