动物传染病流行的三个基本环节是：传染源、传播途径和易感人群。（）
参考答案：对
我国内流河与外流河的汛期主要集中在夏季，其主要影响因素分别是[ ]
A、气温、夏季风
B、夏季风、冬季风
C、冬季风、夏季风
D、夏季风、气温
The girl had hardly rung the bell ___ the door was opened suddenly, and out rushed her friend. [ ]
A. before
B. when
C. until
D. since
