Three taps A, B and C can fill a tank in 10, 18 and 6 hours, respectively. If A is open all the time and B and C are open for one hour each alternatively, starting with B, the tank will be full in:
1
\(6\frac{1}{2}\) hours
2
5 hours
3
6 hours
4
\(5\frac{1}{2}\) hours