1.whether a firm stays in business or shuts down depends hea

1.whether a firm stays in business or shuts down depends heavily on the concept of
a Economic profit
b Actual profit
c Market profit
d Concentration ratios
2.Economic theory would suggest that the profitability of an industry would be
a directly related to the number of firms competing in the industry.
b Inversely related to the number of firms competing in the industry
c Unrelated to the number of firms competing in the industry
d zero in the long run,regardless of market structure.
3 Under perfect competition,the supply curve is
a The marginal cost curve for all price quantity combinations.
b The marginal cost curve,but only that portion that is downward sloping.
c The marginal cost curve,but only that portion that is upward sloping.
d The marginal cost curve,but only that portion that is above the minimum of average variable cost.
4 In a diagram of perfect competition,the marginal revenue line moves up and down when there is exit and entry,respectively,because
a The market demand for the good rises and falls when there is exit and entry,respectively.
b The market demand for the food rises and falls when there is entry and exit,respectively.
c The market supply for the good rises and falls when there is exit and entry,respectively
d The market supply for the good rises and falls when there is entry and exit,respectively
5 If MR>MC,then when an additional unit is sold,the firm's
a profit will be positive
b profit will increase
c profit will be negative
d profit will decrease.

先声明一下,我对经济学了解甚少,答案仅给您参考一下~
1 选A. Economic profit is profit above what the market would allow in perfect competition. In a perfectly competitive situation, making economic profits would automatically attract other businesses until any economic profit is squeezed out of the market
2.大概是D, 完全竞争条件下长期均衡利润应当为0的
3.完全竞争条件下,厂商短期供给曲线就是其边际成本曲线,是一条向右上方倾斜的曲线,即短期边际成本曲线SMC上大于或等于平均可变成本曲线最低点以上的,由此选D似乎更严谨
4.好像也是D 是说有完全竞争市场有厂商进入和退出时边际收益曲线的上升或下降的原因是什么~
5.mr=marginal revenue,mc=marginal cost,即边际收益和边际成本,这题选A