Explain why the following functions $F: \mathbb{R}^2 \rightarrow \mathbb{R}^2$ are not linear.
(a) $\left(\begin{array}{l}x+2 \\ x+y\end{array}\right)$,
(b) $\left(\begin{array}{l}x^2 \\ y^2\end{array}\right)$,
(c) $\left(\begin{array}{l}|y| \\ |x|\end{array}\right)$,
(d) $\left(\begin{array}{c}\sin (x+y) \\ x-y\end{array}\right)$,
(e) $\left(\begin{array}{l}x+e^y \\ 2 x+y\end{array}\right)$.