DFD模型主要由哪几部分组成______。
①外部项 ②数据流
③数据存储和处理 ④箭头和活动

A) ①②
B) ①②③
C) ①②④
D) ①②③④

People often recall having felt chilled before the onset of a cold. This supports the hypothesis that colds are, at least sometimes, caused by becoming chilled; it is the chill that allows a rhino virus, if present, to infect a person. Which of the following, if true, most seriously weakens the force of the evidence cited above

A．Being chilled is a form of stress, and stress lowers the defenses of a person’s immune system, which guards against infection.

B．After a rhino virus has incubated in a person for several days, the first symptom it causes is a feeling of chilliness.

C．People who are tired and then become chilled are more likely to catch severe colds than are people who are chilled without being tired.

D．Some people who catch colds are not sure what it was that allowed them to catch cold.

E．(E) Rhino viruses are not always present in the environment, and so a person could become chilled without catching a cold.