A vertical spring with force constant k is fixed on a table. A ball of mass m at a height h above the free upper end of the spring falls vertically on the spring so that the spring is compressed by a distance d. The net work done in the process is (a) mg(h+ d) = kd - (b) mg(h - d) --kd (c) mg(hd) +=kd (d) mg(h+ d) + = kd 2