以下哪一项不是病例对照研究的优点
A.适用于研究罕见病
B.可在一次调查中调查多个危险因素
C.不需大样本
D.可计算疾病的发病率
E.可从获得的资料中得出一定结论
参考答案:D
For some time the expert __________ people of the health risks caused by passive smoking. He says he won't stop until all the smokers around him decide to quit smoking.
A.is warned
B.is warning
C.has been warning
D.has been warned
金属与周围土壤的()作用而引起的金属破坏即为土壤腐蚀。
A.化学
B.电化学
C.非电解质
D.电解质