Suppose $A$ and $B$ are orthogonal $n\times n$ matrices. Which of the following is also an orthogonal matrix? Assume that $O$ is the null matrix of order $n\times n$ and $I$ is the identity matrix of order $n.$
- $AB-BA$
- $\begin{pmatrix} A & O \\ O & B \end{pmatrix}$
- $\begin{pmatrix} A & I \\ I & B \end{pmatrix}$
- $A^{2}-B^{2}$