以社区为基础的病例对照研究的缺点是（）
A.选择偏倚较大
B.研究对象不容易找
C.研究对象代表性差
D.研究实施困难
E.研究结果不能外推
参考答案：D
用no matter+ 特殊疑问词或however 与 whatever 完成句子。
1. _____ breaks the law should be punished.
2. ____ he goes abroad, he will buy presents for his sister.
3. ____ hard he works, he can not satisfy his boss.
4. _____ difficulty there is, we must work together.
预防院内感染的最重要环节是（）
A.贫血、低蛋白血症
B.糖尿病、尿毒症
C.清创术
D.及时采取特异性免疫疗法
E.切断病原菌传播环节
