Questions 138-139
Four positive charges are fixed at the corners of a square, as shown above. Three of the charges have magnitude Q, and the fourth charge has a magnitude 2Q. Point P is at the center of the square at a distance r from each charge.
139. What is the magnitude of the electric field at point P ?
(A) kQ/r2
(B) 2kQ/r2
(C) 4kQ/r2
(D) 5kQ/r2