Mass and charge are said to be analogous quantities for describing (respectively) gravitational potential energy and electric potential energy. But the analogy isn't perfect. How do mass and charge differ in their effects on potential energy?
a. Mass is always negative, whereas charge can be positive or negative.
b. Mass is always positive, whereas charge can be only negative.
c. Mass and charge are always positive and cannot be negative.
d. Mass is always positive, whereas charge can be positive or negative.