A matrix AAA is idempotent if A2=AA^2 = AA2=A. Which of the following matrices are idempotent?
(1001)\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}(1001)
(0000)\begin{pmatrix} 0 & 0 \\ 0 & 0 \end{pmatrix}(0000)
(1100)\begin{pmatrix} 1 & 1 \\ 0 & 0 \end{pmatrix}(1010)
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)