$20 \cdot 39$ Circle $C$ is tangent to three different lines $l, m, n$. Then, in the following three cases:
(1) The angles between $l$ and $m$, $m$ and $n$, $n$ and $l$ are all $60^{\circ}$;
(2) $l / / m / / n$;
(3) There is another circle $\mathrm{C}^{\prime}$ that is also tangent to $l, m, n$. Which of the following can be true?
(A) Only $(1)$.
(B) Only (1) and (3).
(C) Only (1) and (2).
$(D)(1) 、(2)$ and $(3)$.
[Solution] If we consider that three lines $m, n, l$ form an equilateral triangle, then we can draw the incircle $C$ of the triangle, and also the excircle $C^{\prime}$, as shown in the figure.
Furthermore, (2) is impossible to hold. Therefore, only (1) and (3) hold.
Hence, the answer is $(B)$.