Solve the Problem $r$ And the Circumference $C = 2 \pi r$

Multiple Choice

Solve the problem.
-Suppose that the radius $r$ and the circumference $C = 2 \pi r$ of a circle are differentiable functions of $t .$ Write an equation that relates $\mathrm { dC } / \mathrm { dt }$ to $\mathrm { dr } / \mathrm { dt }$ .

A) $\frac { d C } { d t } = 2 \pi \frac { d r } { d t }$
B) $\frac { d C } { d t } = 2 \pi r \frac { d r } { d t }$
C) $\frac { d C } { d t } = \frac { d r } { d t }$
D) $\frac { d r } { d t } = 2 \pi \frac { d C } { d t }$