Which of the following statements is ALWAYS true for two functions f and g whenever the compositions are defined?
a) (g∘f)(x)=(f∘g)(x) for every x
b) (g∘f)(x) exists only when g is invertible
c) In general, (g∘f)(x)=(f∘g)(x)
d) (g∘f)(x) exists only when f and g have the same domain