Two Charges -2q and -Q Are Placed a Distance a Apart

Two charges -2q and -q are placed a distance a apart. A third charge -q is placed so that the force on it is zero. The third charge

A) cannot be placed on the line joining the first two charges.
B) must be placed on the line joining the first two charges, between the charges, and closer to -2q.
C) must be placed on the line joining the first two charges, between the charges, and closer to -q.
D) must be placed on the line joining the first two charges, not between the charges, and on the -2q charge side.
E) must be placed on the line joining the first two charges, not between the charges, and on the -q charge side.