## Questions 78-79
A point charge of +4.0μC is placed on the negative x -axis 0.20 m to the left of the origin, as shown in the accompanying figure. A second point charge q is placed on the positive x -axis 0.30 m to the right of the origin.
78. If the net electric field at the origin is zero. What is q ?
(A) +9.0μC
(B) +6.0μC
(C) −6.0μC
(D) −9.0μC