为合成镇痛药，化学结构属氨基酮类，镇痛作用与吗啡相当，可以口服的是（）.
A．芬太尼
B．吗啡
C．哌替啶
D．美沙酮
E．纳洛酮
参考答案：D
简述位间隔的识别原理。
基本语法
1. I was ill ,so I __(不得不)stay at home today.
2. You____ (不必)return that book to me at once.
3. We _ (可能去)to visit my father's farm during the summer vacation.
4. You ___ (可以)call him. He __ (不可能)do anything else this afternoon.
5. ______(或许)she will invite you to her party ,too.
