问题如下：
The demand schedule in a perfectly competitive market is given by P = 93 – 1.5Q (for Q ≤ 62) and the long-run cost structure of each company is:
Total cost: 256 + 2Q + 4Q2
Average cost: 256/Q + 2 + 4Q
Marginal cost: 2 + 8Q
New companies will enter the market at any price greater than:
选项：
A.8.
B.66.
C.81.
解释：
B is correct.
The long-run competitive equilibrium occurs where MC = AC = P for each company. Equating MC and AC implies 2 + 8Q = 256/Q + 2 + 4Q.
Solving for Q gives Q = 8. Equating MC with price gives P = 2 + 8Q = 66. Any price above 66 yields an economic profit because P = MC > AC, so new companies will enter the market.
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: 2 + 8Q = 256/Q + 2 + 4Q, Q=8.，
解得： P = 2 + 8Q = 66。
注意到，之所以不用需求函数 P = 93 – 1.5Q 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。
为什么MC=AC是均衡的条件？
