A negatively charged particle is at rest in a region where a uniform magnetic field points in the positive x direction and a uniform electric field points in the positive y direction. What is the initial direction of the vector sum of the forces exerted on the particle?

a. The vector sum of the forces is initially in the negative x direction.
b. The vector sum of the forces is initially in the negative y direction.
c. The vector sum of the forces is initially in the positive x direction.
d. The vector sum of the forces is initially in the positive y direction.
e. The vector sum of the forces is initially in the negative z direction.
f. The vector sum of the forces is initially in the positive z direction.