-I come all the way to say "Thank you" and we ________ it without you.

-Think nothing of it.[ ]

A．should have done  

B．mustn't have done

C．couldn't have done  

D．needn't have done

下列关于病例对照研究的说法正确的是（）

A.比较病例组和对照组发病率的差异

B.比较病例组和对照组死亡率的差异

C.比较暴露组和非暴露组发病率的差异

D.比较暴露组和非暴露组死亡率的差异

E.比较病例组和对照组暴露于某种致病因素的百分比差异