Which of the following functions $F: \mathbb{R}^2 \rightarrow \mathbb{R}^2$ are linear?
(a) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{l}x-y \\ x+y\end{array}\right)$,
(b) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{l}x+y+1 \\ x-y-1\end{array}\right)$,
(c) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{c}x y \\ x-y\end{array}\right)$,
(d) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{l}3 y \\ 2 x\end{array}\right)$,
(e) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{c}x^2+y^2 \\ x^2-y^2\end{array}\right)$,
(f) $F\left(\begin{array}{l}x \\ y\end{array}\right)=\left(\begin{array}{c}y-3 x \\ x\end{array}\right)$.