问题如下：
Which of the following is least likely the reason of an economic loss among the perfect competitive industry?
选项：
A. A collusion agreement is made to lower the current price.
B. Price per unit is lower than average total cost.
C. New entrants increase the total output in the market.
解释：
A collusion agreement is usually made by oligopolistic industry to cheat on something. Perfect competitors are price takers and cannot decide the price of products. If price is lower than average total cost, there will be economic loss. An increase in total output will decrease the price as consumers’ demand remains constant but supplied quantity increases.
考点：完全竞争厂商，合谋。
解析：
合谋是寡头市场的产物，所以A选项入选。
完全竞争对手是价格接受者，不能决定产品的价格。如果价格低于平均总成本，就会有经济损失。总产出的增加将使价格下降，新厂商的进入会瓜分市场利润，压低市场价格，导致新一轮亏损的产生，所以B,C选项不入选。
