f: X--->Y and g: Y----> Z are functions. Prove that

a) if both f and g are injective then g*f is injective

b) if both f and g are surjective then g*f is surjective

c) if both g*f is a bijection, then f is an injection and g is a surjection

( * is that small circle that means one function 'plugged into' into another)