$17 \cdot 41$ In a scalene $\triangle ABC$, the feet of the three altitudes are $D$, $E$, $F$. Then the orthocenter of $\triangle ABC$ is the 
(A) incenter.
(B) circumcenter.
(C) centroid.
(D) orthocenter.
(Shenyang City, Liaoning Province, "Yucai Cup" Junior High School Mathematics Competition, 1988)
[Solution] As shown in the figure, points $H, D, C, E$ are concyclic, $\therefore \quad \angle 3=\angle 4$. Points $A, F, H, E$ are concyclic, $\quad \therefore \angle 1=\angle 2$. Points $A, F, D, C$ are concyclic, $\therefore \angle 1=\angle 4$. Therefore, $\angle 2=\angle 3$. Hence, $A D, B E, C F$ are the angle bisectors of $\angle E D F, \angle D E F, \angle E F D$ respectively.
Thus, $H$ is the incenter of $\triangle D E F$.
Therefore, the answer is (A).