[" Two bodies of masses "m" and "4m" are placed at a "],[" distance "r." The gravitational potential at a point on the "],[" line joining them where the gravitational field is zero "],[" is "],[[" (a) zero "," (b) "-(4Gm)/(r)],[" (c) "-(6Gm)/(r)," (d) "-(9Gm)/(r)]]