1. In $\triangle A B C$, if $A B \cdot B C<0$, then the shape of $\triangle A B C$ is ( ).
(A) acute triangle
(B) right triangle
(C) obtuse triangle
(D) cannot be determined
-1.D.
From $\boldsymbol{A B} \cdot \boldsymbol{B} \boldsymbol{C}<0$, we can only conclude that the exterior angle of $\angle A B C$ is obtuse, and we cannot determine the shape of $\triangle A B C$.