$17 \cdot 29$ Let $AD$ be the altitude from the vertex $A$ to the base $BC$ of isosceles $\triangle ABC$, and let $BM_1, BM_2, \cdots, BM_n$ be the $n+1$ angle bisectors of $\angle ABC$. Their intersections with $AD$ (from $A$ to $D$) are $M_1, M_2, \cdots, M_n$ respectively. Extend $CM_2$ to intersect $AB$ at $E$. Then the positional relationship between line $EM_1$ and line $BM_n$ is
(A) parallel.
(B) intersecting.
(C) perpendicular intersecting.
(D) dependent on $n$ and cannot be determined.
(Anhui Hefei Junior High School Mathematics Competition, 1990)