Some day I happened to find a chatting room in my QQ, there people were chatting in English. I tried to chat with some of them. To my surprised, I found the oral English of some junior students were better than me. I asked them for advice and they told me to practise more on QQ. But every day after that I would spend one hour practise my oral English on QQ. Day by day I learned from many useful words and expressions. With time went by, I found that I could even communicate some college students freely.

吗啡禁用于分娩镇痛，是由于（）。

A.可致新生儿便秘

B.易产生成瘾性

C.易在新生儿体内蓄积

D.镇痛效果差

E.可抑制新生儿呼吸