The doctor told me that I would recover from my illness if I ______ a healthy diet.

A．got through

B．fit in

C．stuck to

D．kept up

队列研究所需观察人数与下列哪项因素无关：（）。

A.人群某病发病率水平

B.暴露组与对照组发病率之差

C.显著性水平

D.抽样方法