The potential energy of a particle that moves along the x-axis is $U(x) = 3x^2 - 2x^3$, where $U$ is in joules and $x$ is in metres. Each graph in the options shows a curve of force against position, with one point marked S as the point of stable equilibrium. Which graph correctly shows the force $F$ on the particle and the right point of stable equilibrium?

Graph (a)Graph (b)Graph (c)Graph (d)