Given a potential energy function U(x) , the corresponding force F ? is zero if:
a) U has a maximum at the point x.
b) U is positive and decreasing.
c) U is negative and increasing.
d) U is positive and increasing.
e) U has a minimum at the point x.
f) U is zero and constant.
g) U is negative and constant.
h) U is zero and increasing.
i) U is negative and decreasing.
j) U is positive and constant.
k) U is zero and decreasing.