Write an Equation to Describe the Variation $s$ Varies Directly As $t$

Multiple Choice

Write an equation to describe the variation. Use k for the constant of proportionality.
- $s$ varies directly as $t$ and inversely as $u$ .

A) $s = \frac { k t } { u }$
B) $s = \frac { k u } { t }$
C) $\mathrm { s } + \mathrm { t } - \mathrm { u } = \mathrm { k }$
D) $s t u = k$