问题如下：
For a market of perfect competition, the demand schedule is defined as P = 165– 3Q (for Q ≤ 55). For each company in the long-run, the total cost is given by: 125 + Q + 5Q2, the average cost is given by: 125/Q + 1 + 5Q, the marginal cost is given by: 1 + 10Q. At which of the following price, new competitors are most likely to enter?
选项：
A.135.
B.51.
C.45.
解释：
B is correct
In a perfectly competitive market, for each company in the long-run, only when MC = AC = P, would the equilibrium be expected.
we can draw the equation:
MC=AC: 1 + 10Q = 125/Q + 1 + 5Q, Q=5.
At that time, P=MC= 1 + 10Q = 51
If the price is higher than 51, new competitors will enter for economic profits .
考点：完全竞争市场
解析：在完全竞争市场中，存在条件 MC = AC = P，
联立MC=AC: 1 + 10Q = 125/Q + 1 + 5Q, Q=5.，
解得：P=MC= 1 + 10Q = 51。
注意到，之所以不用需求函数 P = 165– 3Q参与联立是因为这里给的是整个市场的需求函数，而非单个厂商的 。但是题目给定的MC,AC都是单个厂商的关系式，所以整个市场的需求函数与它们并不匹配，不能参与联立。
关于这道题目我觉得现在已经提出的问题的回答是不是有错误啊？在价格高于51时，既答案A 135，才会有新的厂商进入，所以才是new competitors most likely to enter。 然后因为有大量厂商进入，价格下降，使得MC=AC=P，市场达到均衡，profit=0，之后就没有新的厂商原因进入了，用MC=AC来计算出的结果只能用于判断哪个价格是break even point, 然后才找出高于break even point 的价格，意味在short term有超额利润，能够吸引新厂商进入，如果价格已经是break even point了，新厂商就没有必要进入了呀，因为已经没有超额利润了。
