Four equal-magnitude point charges, two positive and two negative, are arranged at the corners of a square. Let V and E be the potential and the electric field magnitude, respectively, at the center of the square, and let V0 and E0 be the same quantities at the same point if the upper left positive charge only were present. Which of the following expresses the values of V and E?

A. V = V0, E = E0
B. V = 4 V0, E = 0
C. V = 0, E = 4 E0
D. V = 4 V0, E = 4 E0
E. V = 0, E = 0