If the potential energy function is given by U(r) = br⁻³/² + c, where b and c are constants, which of the following is an expression for the force on the particle?
a) F(r) = -3/2br⁻⁵/²
b) F(r) = -br⁻³/²
c) F(r) = -br⁻¹/²
d) F(r) = -3br⁻¹/²