The Potential Energy of an Object Is Given by U(x)=

The potential energy of an object is given by U(x) = x⁴ - x²,where U is measured in joules and x is measured in meters.At which position is the object in unstable equilibrium?

A) only at 0
B) only at 0.707 m
C) only at -0.707 m
D) at both +0.707 m and -0.707 m
E) There isn't a location where the object is in unstable equilibrium.