One pipe can empty a tank in 3 hours. A second pipe takes 4 hours to complete the same job. How long will it take to empty the tank if both pipes are used?

One pipe can empty a tank in 3 hours. A second pipe takes 4 hours to complete the same job. How long will it take to empty the tank if both pipes are used?

A. 1 and 5/7 hours C. 2 and 1/4 hours
B. 1 and 3/4 hours D. 2 and 3/4 hours