Let $T$ be the set of columns of the matrix $B$ below. Define $W=\spn{T}$. Find a set $R$ so that (1) $R$ has 3 vectors, (2) $R$ is a subset of $T$, and (3) $W=\spn{R}$. \begin{equation*} B= \begin{bmatrix} -3 & 1 & -2 & 7\\ -1 & 2 & 1 & 4\\ 1 & 1 & 2 & -1 \end{bmatrix} \end{equation*}