23. We are told that the industry is in long run equilibrium, and since we know D1 is the demand curve, then S1 must be the industry supply curve, so that P=MC=ATC, as is required for long run equilibrium. Thus, market price would be P1 with each firm producing quantity Q1.

If demand shifts from D1 to D2 price and quantity will increase in the short run. As shown to the right price will increase to P2 and quantity to Q2.

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