Which of the following is a diagonal matrix.

• A. A= ( begin{bmatrix}0&2&1 2&0&1 2&1&0 end{bmatrix} )
• B. A= ( begin{bmatrix}5&1&0 0&5&0 0&0&5 end{bmatrix} )
• C. A= ( begin{bmatrix}4&0&0 0&5&0 0&0&9 end{bmatrix} )
• D. A= ( begin{bmatrix}2&2&2 3&3&3 4&4&4 end{bmatrix} )