Which of these matrices represents a pure shear along the y-axis?
(10k1)\begin{pmatrix} 1 & 0 \\ k & 1 \end{pmatrix}(1k01)
(1k01)\begin{pmatrix} 1 & k \\ 0 & 1 \end{pmatrix}(10k1)
(k001)\begin{pmatrix} k & 0 \\ 0 & 1 \end{pmatrix}(k001)
(100k)\begin{pmatrix} 1 & 0 \\ 0 & k \end{pmatrix}(100k)