里面说，“In a perfectly competitive market, a firm will not earn economic profits for any significant period of time. The assumption is that new firms will enter the industry to earn profits, increasing market supply and eventually reducing market price so that it just equals a firm’s average total cost.” 但是前面不是说perfect competitive market的demand curve是水平线吗？那么新增企业又怎么能使市场价格下降？

而且根据figure 3, MC=ATC 时的quantity要比profit maximizing output的quantity小，但是到了figure 4，quantity增加了，却反而达到了MC=ATC，这是怎么回事？