Subject: mathqa
Question: an electric pump can fill a tank in 3 hours . because of a leak in the tank , it took 3 hours 30 min to fill the tank . in what time the leak can drain out all the water of the tank and will make tank empty ?
Option: 
A: 25 hours
B: 13 hours
C: 21 hours
D: 10 hours
Answer: C