A cistern has 3 pipes, a, b, and c. Pipes a and b are inlet pipes whereas c is an outlet pipe. Pipes a and b can fill the cistern separately in 3 hours and 4 hours. If both pipes are opened simultaneously but pipe c remains open as well, how long will it take to fill the cistern?
a) Less than 3 hours
b) More than 3 hours but less than 4 hours
c) Exactly 3 hours
d) More than 4 hours