Two particles of masses m and M are initially at rest at infinite distance. Find their relative velocity of approach due to gravitational attraction when d is their separation at any instant.

• A

  $\sqrt{\frac{2G(M+m)}{d}}$

• B

  $\sqrt{\frac{G(M+m)}{d}}$

• C

  $\sqrt{\frac{G(M+m)}{2d}}$

• D

  $\sqrt{\frac{G(M+m)}{4d}}$