影响传染病流行过程的因素是（）。
A.自然因素
B.社会因素
C.自然因素和社会因素
D.病原体的毒力
E.病原体的侵袭力
参考答案：C
咳嗽剧烈时，痰中带血（）。
A.肺结核
B.肺癌
C.支气管扩张
D.急性左心衰竭
E.支原体肺炎
I _____ my homework while my parents _____ TV last night. [ ]
A. did; have watched
B. was doing; were watching
C. had done; were watching
D. would do; were watching
