5. $\vec{a}+\vec{b}+\vec{c}=\overrightarrow{0}$ is the ( ) for $\vec{a} 、 \vec{b} 、 \vec{c}$ to form a triangle.
A. Sufficient but not necessary condition
B. Necessary but not sufficient condition
C. Sufficient and necessary condition
D. Neither sufficient nor necessary condition
5. B.
When $\vec{a}, \vec{b}, \vec{c}$ can form a triangle, the three vectors are connected head to tail, then $\vec{a}+\vec{b}+\vec{c}=\overrightarrow{0}$; conversely, if $\vec{a}=\vec{x}, \vec{b}=-2 \vec{x}$, $\vec{c}=\vec{x}, \vec{a}+\vec{b}+\vec{c}=\overrightarrow{0}$, it is clear that they cannot form a triangle.