Two particles of masses m and M are initially at rest and infinitely separated. At a later instant when they are at a finite distance d from each other, their relative velocity of approach is

A

$$\left[\frac{Gm}{2d}\right]^{1/2}$$

B

$$\left[\frac{2G(m + M)}{d}\right]^{1/2}$$

C

$$\left[\frac{G(m + M)}{2d}\right]^{1/2}$$

D

$$\left[\frac{2GM}{d}\right]^{1/2}$$