Two point masses M and 4M are fixed a distance d apart on a straight line. At which region along the line will a small test mass experience zero net gravitational field?
(a) Exactly at the midpoint between the two masses (d/2)
(b) On the line segment between them, at a point two-thirds of the way from M (i.e. closer to 4M)
(c) On the line segment between them, at a point one-third of the way from M (i.e. closer to M)
(d) At a point on the extension of the line, outside the segment on the side of the smaller mass M