$\Lambda$ rock of mass $m$ slides down with an initial velocity $v_{0}$. $\Lambda$ retarding force $F-k v^{12}$ acts on the body, the velocity at any instant is given by
(a) $v=v_{e}-\frac{k t}{m}$
(b) $v=v_{o}-\left(\frac{k t}{m}\right)^{2}$
(c) $\sqrt{v}=\sqrt{v_{e}}-\frac{k t}{2 m}$
(d) Nonc