The potential energy of a particle in a force field is :$U=\frac{A}{r^2}-\frac{B}{r}$ where A and B are positive constants and r is the distance of particle from the centre of the field. For stable equilibrium, the distance of the particle is

Question

( A ) $\frac{A}{B}$
( B ) $2\frac{A}{B}$
( C ) $\frac{B}{A}$
( D ) $\frac{B}{2}A$