If $A = O, \text{tr}(A) = 0$. suppose $A \neq O$ and $A = \begin{bmatrix} a & b \\ c & d \end{bmatrix} \Rightarrow |A| = 0$ and $A^2 - (a + d)A + (ad - bc)I = 0 \Rightarrow a + d = 0$
Correct Answer:B
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