Which of the following expressions gives the correct relationship between the value of a conservative force F(x) that is a function only of position x and its associated potential energy U(x)?
a) F(x) = -dU(x)/dx
b) F(x) = dU(x)/dx
c) F(x) = -d²U(x)/dx²
d) F(x) = d²U(x)/dx²