Which of the following functions $F: \mathbb{R}^3 \rightarrow \mathbb{R}$ are linear? (a) $F(x, y, z)=x$,
(b) $F(x, y, z)=y-2$, (c) $F(x, y, z)=x+y+3$, (d) $F(x, y, z)=x-y-z$,
(e) $F(x, y, z)=x y z$,
(f) $F(x, y, z)=x^2-y^2+z^2$,
(g) $F(x, y, z)=e^{x-y+z}$.