Write the Statement as a Power Function Equation $\mathrm { S }$

Multiple Choice

Write the statement as a power function equation. Use k as the constant of variation.
-The surface area of a sphere $\mathrm { S }$ varies directly as the square of its radius $\mathrm { r }$ .

A) $S = \frac { k } { r ^ { 2 } }$
B) $S = \frac { r ^ { 2 } } { k }$
C) $S = k ^ { 2 } r$
D) $\mathrm { S } = \mathrm { kr } ^ { 2 }$