Let $A$ and $B$ be two $3 \times 3$ matrices such that $( A + B ) ^ { 2 } = A ^ { 2 } + B ^ { 2 }$. Which of the following must be true?\\
(A) $A$ and $B$ are zero matrices.\\
(B) $A B$ is the zero matrix.\\
(C) $( A - B ) ^ { 2 } = A ^ { 2 } - B ^ { 2 }$\\
(D) $( A - B ) ^ { 2 } = A ^ { 2 } + B ^ { 2 }$