Two point charges, +q and −2q, are held a fixed distance r apart in vacuum. Which statement correctly describes the sign of the electric potential energy U of this two-charge system?
a) U is positive because the product q1q2 is taken using magnitudes.
b) U is zero because the electric potentials of the two charges cancel each other.
c) U is negative because the charges have opposite signs.
d) The sign of U cannot be determined without knowing the separation r."