(1) Which of the following sets is a subspace of R³? Why? \begin{aligned} &\text { (a) }\\ &\left\{\left(\begin{array}{c} 3 s+t \\ 4 \\ s-t \end{array}\right): a, b \in \mathbf{R}\right\} \end{aligned}

\begin{aligned} &\text { (b) }\\ &\left\{\left(\begin{array}{l} a-b \\ b-c \\ c-a \end{array}\right): a, b, c \in \mathbb{R}\right\} \end{aligned}