$17 \cdot 121$ In $\triangle A B C$, $\angle B A C=120^{\circ}$, point $P$ is a point inside $\triangle A B C$, then
(A) $P A+P B+P C < A B+A C$.
(C) $P A+P B+P C=A B+A C$.
(D) The size relationship between $P A+P B+P C$ and $A B+A C$ depends on the position of point $P$. (China Guangzhou, Wuhan, Fuzhou, etc. five cities junior high school mathematics league;
China some provinces and cities junior high school mathematics correspondence competition, 1984)