A Conical Tank (With Vertex Down) Is Feet Across the Top

Multiple Choice

A conical tank (with vertex down) is feet across the top and feet deep. If water is flowing into the tank at a rate of cubic feet per minute, find the rate of change of the Depth of the water when the water is feet deep.

A) $\frac { 9 } { 40 \pi } \mathrm { t } / \mathrm { min }$
B) $\frac { 9 } { 100 \pi } \mathrm { f } / \mathrm { min }$
C) $\frac { 81 } { 20 \pi } \mathrm { tt } / \mathrm { min }$
D) $\frac { 81 } { 50 \pi } \mathrm { ft } / \mathrm { min }$
E) $\frac { 81 } { 200 \pi } \mathrm { ft } / \mathrm { min }$