A draining pipe can drain a tank in 12 hours, and a filling pipe can f

A draining pipe can drain a tank in 12 hours, and a filling pipe can fill the same tank in 6 hours. A total of n pipes which include both types of pipes can fill the entire tank in 2 hours. Which of the following could be a value of n?

I. 6
II. 7
III. 9

A. I only
B. II only
C. I and II only
D. I and III only
E. I, II, and III


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