A water tap fills a tub in 'p' hours and a sink at the bottom empties it in 'q' hours. If p < q and both tap and sink are open, the tank is filled in 'r' hours; then 

A

$$ \frac{1}{r} = \frac{1}{p} + \frac{1}{q}  $$

B

$$ \frac{1}{r} = \frac{1}{p} - \frac{1}{q}  $$

C

$$ r = p + q$$  

D

$$ r = p - q$$