If A = [a_{ij}] is an identity matrix, then which of the following is true ? (A) a_{ij} = \begin{cases} 0, & \text{if } i = j \\ 1, & \text{if } i \neq j \end{cases} (B) a_{ij} = 1, \forall i, j (C) a_{ij} = 0, \forall i, j (D) a_{ij} = \begin{cases} 0, & \text{if } i \neq j \\ 1, & \text{if } i = j \end{cases}