调幅信号是指（）.
A、以低频信号调制载波的振幅，使载波振幅随低频信号的瞬时值变化，这种调制方式叫调幅。
B、以低频信号调制载波的频率，使载波频率随低频信号的瞬时值变化，这种调制方式叫调幅。
C、以高频信号调制载波的振幅，使载波振幅随高频信号的瞬时值变化，这种调制方式叫调幅。
D、以低频信号调制载波的振幅，使低频信号振幅随载波的瞬时值变化，这种调制方式叫调幅。
参考答案：A
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| Recently, I stopped by a convenience store to get a newspaper and a bottle of drink. The young woman at the check-out 1 said. "That'll be five dollars in all please." She then 2 down at the paper I was buying and said, "I'm 3 all these negative (消极的) words on the front pages. I want to read some 4 news for a change." She then said, " 5 , I think someone should just 6 a good news newspaper-a paper with wonderful, inspirational 7 about people overcoming difficulty and doing good things for others." She 8 me for coming in and said, "Maybe we'll get some good news," and she 9 . She made my day. The following day after my business appointments, I 10 the same store again to pick up bottled water and a piece of newspaper, but a 11 young lady was behind the counter. As I checked out I said, "Good afternoon" and 12 her my money. She said nothing-not a word, not a smile or not a 13 . She just gave me my change and 14 a negative tone ordered "Next!" It 15 me right between the eyes: Two people, the same age; one made me feel 16 , but the other, well, made me feel that I had inconvenienced her by 17 . Every morning, you should ask 18 this important question: "Who do I want to be today? 'The Grouch (不 高兴的人) 'or' The Good News Girl'"? Your answer will go a long way toward 19 the joy and happiness that you will experience in your 20 . | ||||
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