$20 \cdot 102$ The ratio of the areas of two concentric circles is $1: 3$. If the radius of the smaller circle is $r$, then the best approximation of the difference in radii is
(A) $0.41 r$.
(B) 0.73 .
(C) 0.75 .
(D) $0.73 r$.
(E) $0.75 r$.
(6th American High School Mathematics Examination, 1955)
[Solution] Let the difference in radii of the two circles be $d$. According to the problem, $\frac{\pi(r+d)^{2}}{\pi r^{2}}=\frac{3}{1}$, which means $\square$
$$
\frac{r+d}{r}=\sqrt{3} \text {. }
$$

Solving for $d$ yields $d=0.732 r$.
Therefore, the answer is $(D)$.