A cistern can be filled by two pipes A and B in 4 hours and 6 hours respectively. When full, the tank can be emptied by a third pipe C in 8 hours . If all the taps be turned on at the same time, the cistern will be full in?

• 3 hours 18 min.
• 3 hours 26 min.
• 3 hours 42 min.
• 3 hours 48 min.