1. If vector $\overrightarrow{A B}=(3,-1), \boldsymbol{n}=(2,1), \boldsymbol{n} \cdot \overrightarrow{A C}=7$, then $\boldsymbol{n} \cdot \overrightarrow{B C}=$
A. -2
B. 2
C. -2 or 2
D. 0
1. $\mathrm{B}$ Hint: Since $\overrightarrow{B C}=\overrightarrow{A C}-\overrightarrow{A B}$, therefore, $\boldsymbol{n} \cdot \overrightarrow{B C}=\boldsymbol{n} \cdot(\overrightarrow{A C}-\overrightarrow{A B})=\boldsymbol{n} \cdot \overrightarrow{A C}-\boldsymbol{n} \cdot \overrightarrow{A B}=7-(3 \times$ $2-1 \times 1)=7-5=2$