4. Given $\vec{a}=-\vec{i}+2 \vec{j}, \vec{b}=2 \vec{i}-3 \vec{j}$, and $\vec{a} \cdot \vec{c}=-7, \vec{b} \cdot \vec{c}=12, \vec{c}$ equals ( ).
A. $-3 \vec{i}-2 \vec{j}$
B. $2 \vec{i}-3 \vec{j}$
C. $3 \vec{i}-2 \vec{j}$
D. $-3 \vec{i}+2 \vec{j}$
4. C. Let $\vec{c}=\{x, y\}$, from the problem we get $-x+2 y=-7, 2 x-3 y=12$, thus $x=3, y=-2$.