Answer
$C$ varies directly as $r$, and the constant of this variation is $2\pi$
Work Step by Step
The general equation for direct variation is $y=kx$, where k is the constant of the variation.
Here, $C=2\pi r$, means that $C$ varies directly with $r$.
The equation for direct variation here, is $C=k\times r$
As $2\pi$ is constant, this will be the constant of the variation.
Therefore the circumference of a circle varies directly with its radius, and the constant of this variation is $2\pi$.
