If $z_1+z_2+z_3=0$ and $\mid z_1\mid=\mid z_2\mid=\mid z_3\mid=1$ then value of $z_1^2+z_2^2+z_3^2$ equals
$\begin{array}{1 1}(A)-1 \\ (B) 0 \\ (C) 1 \\(D) 3 \end{array}$