$20 \cdot 32$ If four times the reciprocal of the circumference of a circle equals the diameter of the circle, then the area of the circle is
(A) $\frac{1}{\pi^{2}}$.
(B) $\frac{1}{\pi}$.
(C) 1 .
(D) $\pi$.
(E) $\pi^{2}$.
(29th American High School Mathematics Examination, 1978)
[Solution] If $r$ and $A$ are the radius and area of a circle, respectively, then $\frac{4}{2 \pi r}=2 r$, which means $4=4 \pi r^{2}$, so $1=\pi r^{2}=A$. Therefore, the answer is $(C)$.