12. Two particles with charges +Q and -Q are fixed at the vertices of an equilateral triangle with sides of length a. The work required to move a particle with charge +q from the third vertex to the center of the line joining the fixed particles is: A) 0 B) keQq/a C) keQq/a D) 2kQq/a E) 2kQq/a +Q a a +q a