搜题请搜索小程序“爱下问搜题”
问题 选择题

关于多普勒效应的叙述，下列说法中正确的是(　　)

A．多普勒效应说明波源的频率发生了变化

B．多普勒效应是由于波的干涉引起的

C．多普勒效应是波共有的特征

D．多普勒效应是由于观察者与波源的相对运动引起的

答案

CD

多普勒效应是波共有的特征，是由于观察者与波源的相对运动引起的接收者频率变化而产生的，而波源的频率并没有发生变化，CD正确。

单项选择题

(一)


Right is right. Right Of course. But is left wrong Well, the Romans used to think so. They thought left-handed people were mistakes of nature. Latin, the language of the Romans, had many Words that expressed this view. Some words we use today still have this meaning. The Latin word "dexter" means "right". The English word "dexterous" comes from this word. It means "handy" ( clever with hands). So, right is handy. But the Latin word for left is "sinister". The English word "sinister" comes from this word. Sinister means "evil" (very bad). Is it fair to call right-handed people handy and: left-handed people evil Well, fair or not, many languages have words that express such beliefs. In old English, the word for left means "weak". That isn’t much of an improvement over "evil".
Not very long ago, children were often forced to write with their right hands. Doctors have since found that this can be very harmful. You should use the hand you were born to use.
People who use their left hands are just starting to get better treatment. But why they get all these bad names in the first place One reason may be that there are not as many left-handed people as there are right-handed people. There is one left-handed person for every five right- handed ones. People who are different are often thought to be wrong. But attitudes do seem to be changing. Fair-minded right-handed people are finally starting to give left-handed people a hand.

Which of the following is NOT right according to the article

A．At one time, left-handed people were forced to use their right hands.

B．Left-handed people today are being treated better than those in the past.

C．There are not as many left-handed people as there are right-handed people today.

D．"Dexterous" comes from Latin, means "evil" and English "sinister" means "handy".
查看答案
单项选择题

塑料电工套管穿过B1级以下(含B1级)的装修材料时，应采用（）材料或防火封堵密封件严密封堵。

A、A级

B、B1级

C、B2级

D、B3级
查看答案