Pipes A and B can fill a tank in 3 and 4 hours. Pipe C can empty it in 4 hours. If all pipes are opened together, then the tank will be filled in?
Choose the correct choice
A) 3 hr
B) 1 / 2 hr
C) 1 hr
D) 9 / 7 hr
E) 5 / 3 hr

Rationale: "net part filled in 1 hour = 1 / 3 + 1 / 4 - 1 / 4 = 1 / 3 the tank will be full in 3 hr answer is a"
Answer: A