which of the following functions is not linear?

a)for all x€R,f(x)=(x; 2x)

b) for all (x; y; z)€R3,f(x; y; z)=x-2y+3z

c) for all (x; y) € R2,f(x; y)= (x+1; y-1)

d) they're all linear

And why???

I hope it's clear enough to understand...These things are driving me crazy, any help is really appreciated..thanks