.Among all the problems,_______is so difficult as this one. 

In societies where social roles are determined, boys usually copy the behavior of their fathers, and girls usually also copy _____ of their mothers.

Of all my friends,__________ is so diligent as Tom.

—Could I smoke here?—I’d prefer _______if you don’t smoke in front of the children.

In American there are more households(户主) with pets than ____ with children.