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问题 选择题

一正弦交流电的电压随时间变化的规律如图所示。由图可知该交流电电压瞬时值的表达式为

A．

B．

C．

D．

答案

答案：A

题目分析：从图可以知道，该交变电流的电压最大值为Um=311v，该交变电流的周期为T=0.02s，则交变电流的电源线圈角速度为w=2π/T=100π，据交变电流的电压瞬时值表达式u=Umsinwt，所以A选项正确而B、C、D选项错误。

填空题


In the following passage, there are 20 blanks representing the words that are missing from the context. You are to put back in each of the blanks the missing worD．The time for this section is 20 minutes.
Many things about language are a mystery, and many will always remain so. But some things we do know.
First, we know that all human beings have a language of some sort. (1) is no race of men anywhere on earth so backward that it has (2) language, no set of speech sounds by which the people communicate with one (3) . Furthermore, in historical times, there has never been a race of men (4) a language.
Second, there is no such thing as a primitive language. There are many people (5) cultures are underdeveloped, who are, as we say, uncivilized, but the (6) they speak are not primitive. In all known languages we can see complexities (7) must have been tens of thousands of years in development.
This has not (8) been well understood; indeed, the direct contrary has often been stateD．Popular ideas (9) the language of the American Indians will illustrate. Many people have supposed that the Indians (10) in a very primitive system of noises. Study has proved this (11) be nonsense. There are, or were, hundreds of American Indian languages, and all of them (12) out to be very complicated and very old．They are certainly (13) from the languages that most of us are familiar with, but they are (14) more primitive that English and Greek.
A third thing we know about language (15) that all languages are perfectly adequate. That is, each one is a perfect (16) of expressing the culture of the people who speak the language.
Finally, we (17) that language changes. It is natural and normal for language to change; the (18) languages which do not change are the dead ones. This is easy to (19) if we look backward in time. Change goes in all aspects of language. (20) features change as do speech sounds, and changes in vocabulary are sometimes very extensive and may occur very rapidly. Vocabulary is the least stable part of any language.

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填空题

呼吸频率为_____________次/分时，所消耗的功或能量为最少，这是呼吸管理所需求的频率。
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