Determine which matrix is idempotent (A2=AA^2 = AA2=A).
(1001)\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}(1001)
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)
(2002)\begin{pmatrix} 2 & 0 \\ 0 & 2 \end{pmatrix}(2002)
(1200)\begin{pmatrix} 1 & 2 \\ 0 & 0 \end{pmatrix}(1020)