If $\vec{a}, \vec{b}, \vec{c}$ are unit vectors such that $\vec{a}+\vec{b}+\vec{c}=\vec{0}$, then the value of $\vec{a} . \vec{b}+\vec{b} . \vec{c}+\vec{c} . \vec{a}$ is:

$\frac{1}{2}$ $\frac{3}{2}$ $-\frac{3}{2}$ 0

$-\frac{3}{2}$

The correct answer is Option (3) → $-\frac{3}{2}$