问题
单项选择题
很多呼吸道传染病都具有周期性流行的规律，其主要原因是（）。
A．易感者的积累
B．病原体的变异
C．人口流动
D．发病率下降
E．以上都不是
答案
参考答案：A
搜题请搜索小程序“爱下问搜题”
很多呼吸道传染病都具有周期性流行的规律，其主要原因是（）。
A．易感者的积累
B．病原体的变异
C．人口流动
D．发病率下降
E．以上都不是
参考答案：A
| 从方框中选择最佳选项补全对话，其中有两项是多余的。 | |
R: Hello. Susan 1 S: I'm with a team of explorers (考察者) and scientists. We're having a trip to the North Pole (北极圈). R: 2 S: On foot. R: 3 S: It's a long way. We're walking over 1,000 kilometers! R: 4 S: It's very, very cold. During the day it's often sunny and windy, too. R: 5 S: Well, we're all wearing very warm clothes, so it's OK. R: Of course, there aren't any hotels in the North Pole. Where do you sleep? S: We've got special tents and sleeping bags. The temperature is—35C at night. R: Oh, it's awful! Thank you, Susan. Goodbye. S: Goodbye. 1._____ 2._____ 3._____ 4._____ 5._____ |