Infinite numbers of masses, each of mass m,are placed along straight line at distances of r,2r,4r,8r etc from a reference point O.Find the gravitational field at point O

Question

$$(a)\;\frac{2GM}{r^2} \quad (b)\;\frac{4GM}{3r^2} \quad (c)\;\frac{4GM}{r^2} \quad (d)\;\frac{2GM}{3r^2}$$