就流行病学实验研究而言,下述哪项是正确的
A.属于观察性研究
B.研究方向由果及因
C.在实验室中进行
D.随机分组、实施干预措施
E.评价指标是RR或OR
参考答案:D
— I didn't do well in the exam yesterday. Of course I ______ for it.
— Oh, your poor thing! But I think you should learn a lesson. [ ]
A. hadn't prepared
B. haven't prepared
C. didn't prepare
D. wouldn't prepare
下列含有极性键的非极性分子是 ( )
(1)CCl4(2)NH3(3)CH4(4)CO2(5)N2(6)H2S (7)SO2(8)CS2(9)H2O(10)HF
A.(1)(3)(4)(8)
B.(1)(3)(4)(5)(8)
C.(2)(3)(4)(5)(8)
D.以上均不对