For which pairs of functions is f(g(x))=x?
A) f(x)=x^2 and g(x)=1/x
B) f(x)=2/x and g(x)=2/x
C) f(x)x-2/3 and g(x)=2-3x
D) f(x)1/2x-2 and g(x)=1/2x+2