9・38 In $\triangle A B C$, $\sin A \cos C<0$, then this triangle must be
(A) Acute triangle.
(B) Right triangle.
(C) Obtuse triangle.
(D) None of the above answers is correct.
(China Beijing High School Mathematics Competition, 1987)
[Solution] From $00$.
But $\sin A \cos C<0$, so $\cos C<0$.
Since $\angle C$ is an interior angle of $\triangle ABC$, $\cos C<0$ indicates that $\angle C$ is an obtuse angle. Therefore, $\triangle ABC$ is an obtuse triangle.
Hence, the answer is $(C)$.