Look at the picture. We are supposed to____________.

A．show our love to our parents

B．protect the environment

C．work hard on our study

D．say thanks to our teachers

与病例对照研究相比，队列研究的主要优点是（）

A.省钱省时省人力

B.易于获得研究对象

C.因果顺序合理

D.研究周期较短

E.适于罕见病的研究