Write an Equation to Describe the Variation $x$ Varies Jointly As $y$

Multiple Choice

Write an equation to describe the variation. Use k for the constant of proportionality.
- $x$ varies jointly as $y$ and $z$ .

A) $x = k y z$
B) $x = k + y + z$
C) $x y z = k$
D) $x + y + z = k$