Volume and Radius $8 \mathrm { in. } ^ { 3 } / \mathrm { min }$

Volume and radius. Suppose that air is being pumped into a spherical balloon at a rate of $8 \mathrm { in. } ^ { 3 } / \mathrm { min }$ At what rate is the radius of the balloon increasing when the radius is 3 in.?

A) $\frac { d r } { d t } = \frac { 8 } { 9 \pi }$
B) $\frac { d r } { d t } = \frac { 2 } { 3 \pi }$
C) $\frac { d r } { d t } = \frac { 9 } { 8 \pi }$
D) $\frac { d r } { d t } = \frac { 3 } { 8 \pi }$
E) $\frac { d r } { d t } = \frac { 2 } { 9 \pi }$