20.61 As shown in the figure, each of the three circles is externally tangent to the other two, and each side of the triangle is tangent to two of the circles. If the radius of each circle is 3, then the perimeter of the triangle is 
(A) $36+9 \sqrt{2}$.
(B) $36+6 \sqrt{3}$.
(C) $36+9 \sqrt{3}$.
(D) $18+18 \sqrt{3}$.
(E) 45.
[Solution] As shown in the figure, let $P B$ and $Q C$ be the radii from the centers $P$ and $Q$ to the points of tangency $A D$ of the common tangent.
$$
\begin{aligned}
\because \quad \angle P A B & =\angle Q D C=30^{\circ}, \\
A B & =C D=3 \sqrt{3},
\end{aligned}
$$

and
$A D=A B+P Q+C D=6+6 \sqrt{3}$.
$\therefore$ The perimeter of the triangle is $18+18 \sqrt{3}$.
Therefore, the answer is $(D)$.