9. The complex numbers $z_{1}, z_{2}, z_{3}, z_{4}$ satisfy $\left|z_{1}\right|=\left|z_{2}\right|=\left|z_{3}\right|=\left|z_{4}\right|=1$, and $z_{1}+z_{2}+z_{3}+z_{4}=0$. What shape is the quadrilateral formed by the points corresponding to these four complex numbers? Explain the reason.
9. Rectangle, hint: $z_{1}, z_{2}, z_{3}, z_{4}$ are on the unit circle. And the midpoint of $z_{1}, z_{3}$ coincides with the midpoint of $z_{2}, z_{4}$.