10. A spherical ball of ice of radius $20 m$ is dropped in a vat of hot water. The ice melts in such a way that (i) the shape of the ball remains spherical, and (ii) the radius of the ball decreases at a constant rate of $0.5 m s ^ { - 1 }$. At what rate does the volume of the ice ball decrease, when the radius of the ball is $15 m$ ?\\
(a) $100 \pi m ^ { 3 } s ^ { - 1 }$\\
(b) $225 \pi m ^ { 3 } s ^ { - 1 }$\\
(c) $450 \pi m ^ { 3 } s ^ { - 1 }$\\
(d) $600 \pi m ^ { 3 } s ^ { - 1 }$\\