Which matrix represents a pure shear along the x-axis with factor k=3k=3k=3?
\begin{pmatrix} 1 & 3 \ 0 & 1 \end{pmatrix}
\begin{pmatrix} 1 & 0 \ 3 & 1 \end{pmatrix}
\begin{pmatrix} 3 & 0 \ 0 & 1 \end{pmatrix}
\begin{pmatrix} 1 & 0 \ 0 & 3 \end{pmatrix}