Suppose Sal's Pizza Shop has a unique recipe for pizza, and that currently its optimal price is $20 per pizza, at a quantity of 200 pizzas per week. Its marginal cost is $12 per pizza when it produces fewer than 180 pizzas per week. The marginal cost is $15 per pizza when it produces 180 to 210 pizzas per week. The marginal cost is $18 per pizza when it produces between 211 and 300 pizzas per week. The staff cannot produce more than 300 pizzas per week. Assuming that fixed costs are $300 per week, what is its marginal revenue from the 200th pizza sold this week? Explain.