A pipe can fill a tank in ‘x’ hours and another pipe can empty it in ‘y’ hours (y > x). If both the pipes are open, in how many hours will the tank be filled?
[A] (y – x) hours
[B] (x – y) hours
[C] $\frac{xy}{y-x} hours$
[D] $\frac{xy}{x-y} hours$