Over an extended period of time it is possible for potential firms to enter or for existing firms to leave a competitive industry. In the competitive world, shifts in the productive【1】of the economy from any one line of production to others would【2】in this way. The length of the time period【3】for entry and/or exit will vary, depending upon the【4】involved. Some, like hot dog stands, may be easily and quickly【5】. Others, like textile mills, will require a longer planning and construction period. Whatever the time【6】necessary for firms to enter or exit a given industry, we【7】that period as the long run for that industry. For different industries, the long run will encompass different time spans. The force that【8】the entry of new firms into an industry is profit【9】. When the firms in an industry are making profits, they provide the investors in those firms with higher-than-average returns on their investments. Higher-than-average returns seldom go【10】. In the long run they attract new investment and new firms. On the other hand, losses provide the【11】for firms in an industry to leave it. Losses mean that returns to investors in the industry are below the average received by investors economy-wide; however, they do not necessarily【12】that the net income of the firm is【13】. It may or it may not be. But investors will want to withdraw their investments from the loss-incurring industry and【14】in other industries where the returns will be greater. Where profits exist, returns on investment are above average. Where losses occur, returns are below average. So investment is【15】being redirected from less valuable to more valuable uses. (1)