As a rock of mass 4 $\mathrm{kg}$ drops from the edge of a 40 -meter-high cliff, it experiences air resistance, whose average strength during the descent is 20 $\mathrm{N}$ . At what speed will the rock hit the ground?
(A) 10 $\mathrm{m} / \mathrm{s}$
(B) 12 $\mathrm{m} / \mathrm{s}$
(C) 16 $\mathrm{m} / \mathrm{s}$
(D) 20 $\mathrm{m} / \mathrm{s}$