波源振动的频率为f0，波源在介质中运动时，波源的前方介质振动的_爱下问搜题

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问题选择题

波源振动的频率为f₀，波源在介质中运动时，波源的前方介质振动的频率为f₁，波源后方介质振动的频率为f₂，则三者的关系为(　　)．

A．f₁＝f₂＝f₀

B．f₁＝f₂＞f₀

C．f₁＝f₂＜f₀

D．f₂＜f₀＜f₁

答案

答案：D

f₀由波源每秒钟所振动的次数决定，介质振动的频率由波源频率及波源相对介质是否移动来决定．波源运动时，在波源的正前方介质振动的频率高于波源振动的频率．

单项选择题

The term "automatic stabilizers" refers to the fact that: （）

A. with given tax rates and expenditure policies, a rise in national income tends to produce a surplus, while a decline tends to result in a deficit.

B. legislators automatically change the tax structure and expenditure programs to correct upswings and downswings in business activity.

C. government expenditures and tax receipts automatically balance over the course of the business cycle, although they may be out of balance in any single year.

填空题

信息资源管理是一种（），也是一种（）。