长期应用可以引起成瘾的药物是（）。

A.阿司匹林

B.对乙酰氨基酚

C.布洛芬

D.芬太尼

E.哌替啶

Which of the following characteristic of perfect competition explains why firms in perfect competition are referred to as price takers（）

A. Each firm is small relative to the total market.

B. The demand curve is horizontal for firms in perfect competition.

C. There are many independent firms.