If for a matrix $A$, $A^2+I=0$, where $I$ is the identity matrix of order 2, then $A$ can be:

1. A
   $\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]$
2. B
   $\left[\begin{array}{cc}i& 0\\ 0& i\end{array}\right]$
3. C
   $\left[\begin{array}{cc}i& 0\\ 0& 1\end{array}\right]$
4. D
   $\left[\begin{array}{cc}-1& 0\\ 0& -1\end{array}\right]$