Which item is true of f(x) and g(x)?

They are inverses if f(g(x)) = g(f(x)) = -x

They are inverses if f(g(x)) = g(f(x)) = 1

They are inverses if f(g(x)) = g(f(x)) = x

They are inverses if f(g(x)) = g(f(x)) = 0