2. In $\triangle A B C$, if $\overrightarrow{A B} \cdot \overrightarrow{B C}>0$, then $\triangle A B C$ must be
A. Acute triangle
B. Right triangle
C. Obtuse triangle
D. Scalene triangle
2. $\mathrm{C}$ hint, let $\angle A B C=\theta$, then $\overrightarrow{A B} \cdot \overrightarrow{B C}=-\overrightarrow{B A} \cdot \overrightarrow{B C}=-|\overrightarrow{B A}| \cdot|\overrightarrow{B C}| \cdot \cos \theta$, since $\overrightarrow{A B} \cdot \overrightarrow{B C}>0$ so $\cos \theta<0$ so $\angle A B C$ is an obtuse angle.