高斯白噪声通常是指噪声的什么量服从高斯分布( )

A、幅值

B、相位

C、自相关函数

D、功率谱密度

公文区别于其他信息记录的特点是（　　）。

A、具有考查价值

B、具有法定权威与现行效用

C、属于书面文字材料

D、能迅速传播知识信息

根据功率密度谱关系式，一个可用的数字基带信号功率密度谱中必然包含____分量。

SSB信号进行相干解调可获得的制度增益为( )

A、1

B、2

C、2/3

D、小于2/3

请阅读 Passage 2，完成第 26~30小题。Passage 2 The common cold is the world's most widespread illness， which is plague that flesh receives.The most widespread fallacy of all is that colds caused by cold.They are not， They are caused by viruses passing on from person to person.You catch a cold by coming into contact， directly or indirectly， with someone who already has one.If cold causes colds， it would be reasonable to expect the Eskimos to suffer from them forever.But they do not.And in isolated arctic regions explorers have reported being free from colds until coming into contact again with infected people from the outside world by way of packages and mail dropped from airplanes.During the First World War soldiers who spent long periods in the trenches， cold and wet，showed no increased tendency to catch colds.In the Second World War prisoners at the notorious Auschwitz concentration camp， naked and starving， were astonished to find that they seldom had colds.At the Common Cold Research Unit in England， volunteers took part in Experiments in which they gave themselves to the discomforts of being cold and wet for long stretches of time.After taking hot baths， they put on bathing suits， allowed themselves to be with cold water， and then stood about dripping wet in drafty room.Some wore wet socks all day while others exercised in the rain until close to exhaustion.Not one of the volunteers came down with a cold unless a cold virus was actually dropped in his nose.If， then， cold and wet have nothing to do with catching colds， why are they more frequent in the winter? Despite the most pains-taking research， no one has yet found the answer， One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times， and this makes it easier for cold viruses to be passed on.No one has yet found a cure for the cold.There are drugs and pain suppressors such as aspirin， but all they do is relieve the symptoms.The author cites the Eskimos not suffering from common colds forever to indicate ____.

A、common colds are more severe than other plagues

B、viruses are the factors causing common colds

C、common colds are full of myth

D、the idea that cold leads to colds doesn’t stand up

在数字调制通信系统的接收机中，应先采用___同步，其次采用___同步，最后采用___同步。