6. Given $\vec{a}=\{3,-2 m\}, \vec{b}=\{8,3 m\}$, if $\vec{a}$ is perpendicular to $\vec{b}$, then the value of the real number $m$ is ( ).
A. 4
B. 2
C. -4
D. $\pm 2$
6. D. $\vec{a} \perp \vec{b}$, so $\vec{a} \cdot \vec{b}=0$.