Two point charges of equal magnitude, +q and −q, are initially separated by 2.0 cm in vacuum. The charges are then slowly moved apart until the separation is 4.0 cm. Which of the following correctly describes how the electric potential energy U of the system changes?
a) It stays the same because the magnitudes of the charges do not change
b) Its magnitude decreases to one-half of the original value, and U remains negative
c) Its magnitude decreases to one-quarter of the original value, and U remains negative
d) Its magnitude decreases to one-half of the original value, and U becomes positive