please to do solutions by step

Scenario 12.1: Suppose mountain spring water can be produced at no cost and that the demand and marginal revenue curves for mountain spring water are given as follows: Q = 6000 - 5P MR = 1200 - 0.4Q 28) Refer to Scenario 12.1. What is the profit maximizing price of a monopolist? A) $400 B) $600 C) $800 D) $900 E) none of the above