己内酰胺可用水、酸或碱来引发开环，分别按（）、（）和（）机理进行聚合
参考答案：逐步；阳离子；阴离子
A horse is _______ useful animal.
A．a
B．an
C．the
D．/
Sometimes I feel like going to the countryside to _______ the noise, the dirt and the people. [ ]
A. look for
B. get into
C. get away from
D. look forward to
