For small depths d(d≪R) inside the Earth, which of the following best describes the shape of the graph of acceleration due to gravity g plotted against depth d from the surface down to the centre?
(a) g increases linearly with depth, reaching 2g at d=R
(b) g decreases linearly with depth, reaching 0 at d=R
(c) g follows a downward-opening parabola, touching the d-axis at d=R
(d) g decreases linearly but would reach 0 only at d=2R if the line is extrapolated