For the matrix $B$ below, find a set $S$ that is linearly independent and spans the null space of $B$, that is, $\nsp{B}=\spn{S}$. \begin{equation*} B= \begin{bmatrix} -3 & 1 & -2 & 7\\ -1 & 2 & 1 & 4\\ 1 & 1 & 2 & -1 \end{bmatrix} \end{equation*}