Write an Equation to Describe the Variation $r$ Varies Jointly as the Square of S and the Square

Multiple Choice

Write an equation to describe the variation. Use k for the constant of proportionality.
- $r$ varies jointly as the square of s and the square of $t$ .

A) $r = k s ^ { 2 } t ^ { 2 }$
B) $r + s ^ { 2 } + t ^ { 2 } = k$
C) $r = k + s ^ { 2 } + t ^ { 2 }$
D) $r s ^ { 2 } t ^ { 2 } = k$