单项选择题:
An analyst does research about monopolistic petition. Which of the following statements is most accurate Long-run economic profit under monopolistic petition is zero because:（）
A. entry into and exit from the market are possible with fairly low costs.
B. each firm produces a differentiated product.
C. two or more firms have the incentive to collude.
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答案：A
解析: 垄断竞争行业中长期的经济利润也为零，这是因为一旦有经济利润(超额利润)产生，由于进入门槛低，新的企业就会进入这个行业，使长期的经济利润为零。同样，如果一旦有经济亏损，由于退出门槛低，亏损的企业就会退出这个行业，使长期的经济利润仍然为零。