Subject: mathqa
Question: pumps a , b , and c operate at their respective constant rates . pumps a and b , operating simultaneously , can fill a certain tank in 1 / 2 hours ; pumps a and c , operating simultaneously , can fill the tank in 1 / 2 hours ; and pumps b and c , operating simultaneously , can fill the tank in 2 hours . how many hours does it take pumps a , b , and c , operating simultaneously , to fill the tank .
Option: 
A: 1 / 3
B: 1 / 2
C: 1
D: 5 / 6
Answer: D