Three fourths of the bread _______ by Bob, and the rest of the bread _______ left on the table. [ ]

A. was eaten; were  

B. were eaten; was  

C. were eaten; were  

D. was eaten; was

队列研究中，如果随访时间较长，观察人数变动较大时，多采用下列哪种指标表示发病水平（）。

A、年发病率

B、发病密度

C、累积发病率

D、罹患率

E、以上均不对