If $A$ is a $3 \times 3$ non-singular matrix such that $A A ^ { \prime } = A ^ { \prime } A$ and $B = A ^ { - 1 } A ^ { \prime }$, then $B B ^ { \prime }$ equals, where $X ^ { \prime }$ denotes the transpose of the matrix $X$.\\
(1) $B ^ { - 1 }$\\
(2) $\left( B ^ { - 1 } \right) ^ { \prime }$\\
(3) $I + B$\\
(4) $I$