The masses and radii of the earth and moon are M1 , R1 and M2 , R2 respectively. Their centres are a distance d apart. The minimum speed with which a particle of mass m should be projected from a point midway between the two centres so as to escape to infinity is given by
A. 2 [ G ( M1 + M2 )/m d ]¹/²
B. 2 [ G ( M1 + M2 )/ d ]¹/²
C. 2 [ G ( M1 − M2 )/ m d ]¹/²
D. 2 [ G ( M1 − M2 )/d ]¹/²