Which of the Following Are True of Comparing Marginal Revenue

Which of the following are true of comparing marginal revenue to marginal cost?
(i) it reveals the contribution of the last unit of production to total profit
(ii) it is helpful in making profit-maximising production decisions
(iii) it always reveals whether a firm is making an economic profit
(iv) it tells a firm whether its fixed costs are too high

A) (i) and (ii) only
B) (iii) only
C) (ii) and (iii) only
D) (i) , (ii) , (iii) and (iv)