Two Planets Have Masses M and M/2 Respectively

Two planets have masses M and M/2 respectively. The distance between the two planets is r. The relative distance from the first planet where the gravitational force on a third body due to the two planets would be zero is

A) 0.414 r.
B) 0.500 r.
C) 0.586 r.
D) 3.414 r.