One day I was dining with my friend Zhou in McDonald when two young girls came in hand in hand. While enjoyed a hamburger, I found Zhou looking around at something strangely.

I raised my eyes to see what was happening. The two girls are making gestures skillfully. Obviously they were deaf-mutes. I had learned some sign languages ago, so I showed various gestures with Zhou following me in low spirits. We played and laughed cheerfully as if there had no one else present. Suddenly we saw both the two girls glaring at us. Before we could explain to them, they had walked out angrily, left the food unfinished. They thought we were making joke of them. How I regretted!

流行过程的基本条件是

A．患者、病原体携带者、受感染的动物

B．周围性、地区性、季节性

C．散发、流行、暴发流行

D．传染源、传播途径、易感人群

E．自然因素、社会因素