A positive point charge +Q is placed at x = 0 and a negative point charge −Q is placed at x = a. The magnitude of the electrostatic force between the two is F. If another point charge +Q is placed at x = −a, the net force on the charge at the origin (x=0) is,
(a) 2F in the negative x-direction.
(b) F in the positive x-direction.
(c) 5F/4 in the positive x-direction.
(d) 2F in the positive x-direction.