Afourminusfirm concentration ratio measures a. the fraction of an industry's sales accounted for by the four largest firms. b. the fraction of employment of the four largest firms in an industry. c. how the four largest firms became so concentrated. d. the production of any four firms in an industry.

Choose the statement that is incorrect. a. search activity occurs only in markets where there is a shortage. b. when a price is regulated and there is a shortage, search activity increases. c. the time spent looking for someone with whom to do business is called search activity. d. the opportunity cost of a good is equal to its price plus the value of the search time spent finding the good.