Explain, in your own words, why profit maximization happens at the point where MR = MC, and

Question:

Explain, in your own words, why profit maximization happens at the point where MR = MC, and not where MR is greater than MC. Refer to Figure 7.2. in the textbook. Should the 4th unit of output be sold? What about the 5th unit, 6th unit, 7th unit, 8th unit, 9th, or 10th unit? Use your understanding of MR = MC rule to explain.