病例对照研究的缺点是（）
A.不能估计相对危险度
B.需要样本量大
C.研究所需时间长
D.易产生医德问题
E.回忆时易发生偏倚
参考答案：E
It's four o'clock in the afternoon. The boys _ at the pool.[ ]
A. is swimming
B. are swimming
C. swim
根据句意及汉语提示，填写单词。
小题1:Our English teacher has a warm (心).
小题2:There is so much (噪音) in the yard that I can’t go to sleep.
小题3:Twenty students are on the playground. The (其余部分) are in the classroom.
小题4:Madam, you shouldn’t drive a car without a driver’s (执照).
小题5:It’s a nice day; let’s go for a (野餐).
小题6:The cups are (覆盖) with a piece of red cloth.
小题7:A (士兵) walked in, dressed in a black coat.
