Two Point Charges, a and B, Lie Along a Line

Two point charges, A and B, lie along a line separated by a distance L. The point x is the midpoint of their separation.
-Which combination of charges will yield zero electric field at the point x?

A) +1q and -1q
B) +2q and -3q
C) +1q and -4q
D) -1q and +4q
E) +4q and +4q