For which pairs of functions is (f•g)(x)=x?

A.)f(x) = x^2 and g(x)=1/х
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