Total and Marginal Relationships in the Economic Profit Maximising Model

Total and marginal relationships in the economic profit maximising model
The following questions explore the relationships between total and marginal functions in the economic profit maximising (EPM) model.
i. The total revenue function rises over the range of operating activity portrayed in the text. Why does the marginal revenue function decrease?
ii. The total cost function increases over the relevant range of activity. Why does the marginal cost function exhibit its complex behaviour?
iii. In the EPM model, what is the profit maximising level of volume? Explain.