--Do you often hear John ___ in his room?  --Yes. Listen! Now we can hear him ___ in his room.

A．sing; to sing

B．singing; singing

C．sing; singing

D．to sing; singing

传染病的基本特征为

A．有传染性、流行性、地方性和季节性
B．有传染性、免疫性和病原体
C．有传染性、病原体、免疫性和流行性
D．有传染性、传播途径和免疫性
E．有传染性、免疫性和流行性