Explain why firms should not shut down in the short run until price falls below average variable cost. 3. Suppose your economics professor publicly states that she grades on a curve (i.e., the top 10% of the students get an A, the next 20% get a B, the next 40% get a C, the next 20% get a D, and the lowest 10% fail). The whole class could save itself a lot of work by agreeing privately not to study at all for the final exam and just letting the rankings thus far determine the final grades. Why might such an agreement be difficult to maintain and enforce? Which market structure, if any, is this situation most analogous to?