$20 \cdot 75$ For any $\triangle ABC$, let its perimeter, the radius of the circumcircle, and the radius of the incircle be $l, R$, and $r$, respectively. Then
(A) $l>R+r$.
(B) $l \leqslant R+r$.
(C) $\frac{l}{6}<R+r<6 l$.
(D) None of the above (A), (B), (C) is correct.
(China High School Mathematics League, 1983)