I just need D

Suppose f(n) = ((s(n)), and g(n) = ((r(n)). All four functions are positive-valued and monotonically increasing. Prove (using the formal definitions of asymptotic notations) or disprove (by counterexample) each of the following claims: (a) f(n) g(n) = ((s(n) r(n)) (b) if s(n) = 0(g(n)), then f(n) = O(r(n)) (e) if r(n) = O(s(n)), then g(n) = O(f(n)) (d) if s(n) + g(n) = O(f(n)), then f(n) = O(s(n))