In (a)the industry quantity demanded and quantity supplied of fabric are equated at a price of $10 per yard. In (b)the firm faces a horizontal marginal expenditure (边际支出)curve at a price of SI0 per yard of fabric,and chooses to buy 50 yards.In (a) the industry quantity demanded and quantity supplied of fabric are equated at a price of $10 per yard. In (b) the firm faces a horizontal marginal expenditure (边际支出)curve at a price of $l0 per yard of fabric, and chooses to buy 50 yards