Americans who consider themselves ______ in the traditional sense do not usually hesitate to heap criticism in domestic matters over what they believe is oppressive or wasteful.

A．pedestrian

B．penchant

C．patriotic

D．patriarch

传染病流行过程必需的三个环节是：（）

A.病原体、人体及其所处的环境

B.病原体、自然因素、社会因素

C.病原体毒力、数量及适当的入侵门户

D.病原体、传播途径、易感人群

E.传染源、传播途径、人群易感性