Let $A=\left(\begin{array}{ccc}0& 0& -1\\ 0& -1& 0\\ -1& 0& 0\end{array}\right).$ The only correct statement about the matrix $A$ is

1. A
   $A$ is a zero matrix
2. B
   $A^2=I$
3. C
   $A^{-1}$ does not exist
4. D
   $A=(-1)I$ where $I$ is a unit matrix