The circumference of a circle is $a \pi \; units$ and the area of the circle is $x \pi$ sq. units. If $a=x$, then what is the radius of circle ?

Question

$\begin{array}{1 1} (a)\; 3 \\ (b) \; 4 \\ (c)\; 0 \\ (d)\; 2 \end{array}$