下列各项，与病原体致病作用无关的是（）。
A.侵袭力
B.毒力
C.数量
D.大小
E.变异性
参考答案：D
简述奶牛生产瘫痪的病因。
George is going to talk about the geography of his country, but I'd rather he _____ more on its culture. [ ]
A. focus
B. focused
C. would focus
D. had focused
