控制混杂因素作用的方法有（）。
A.多因素分析
B.分层分析
C.选用新病例
D.随访观察
E.匹配
参考答案：A, B, E
I've never read a better book than this one. _____, it is the best one that I have ever read.[ ]
A. Since then
B. In other words
C. At times
D. In some ways
数学模型 (mathematical model)
