— Has Sam finished his homework today, Anne?

— I have no idea. But he ________ it this morning. [ ]

A .did  

B. had done

C. was doing  

D. had done

下列哪项因素与患病率的变化无关（）

A.发病率的升高或下降

B.病死率的升高或下降

C.人口总数自然增加或减少

D.治疗水平的提高或降低

E.存活时间长短