You’d better go there by train. The train ticket is _____ the airplane ticket.

A．as cheap three times as

B．as three times cheap as

C．three times cheaper than

D．three times as cheaper as

下列不属队列研究特点的是（）

A.可直接计算发病率

B.研究人群不稳定时要计算人年发病率（死亡率）

C.每次调查能同时研究多组暴露

D.一次调查可观察多种结局

E.是由"因"至"果"的研究方法