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问题 填空题

常见的脉冲波形有（）、（）、（）、（）等。

答案

参考答案：指数波；方形波；1/2正弦波；1/4正弦波

单项选择题

反风演习结束后，在风流恢复正常后，风流中瓦斯浓度不超过（）时，方可恢复送电。

A、0.5％ 

B、1％ 

C、1.5％ 

D、2.0％
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单项选择题

Month Average Price per Dozen
April $1.26
May $1.20
June $1.08
The table above shows the average (arithmetic mean) price per dozen of the large grade A eggs sold in a certain store during three successive months．If

as many dozen were sold in April as in May, and twice as many were sold in June as in April, what was the average price per dozen of the eggs sold over the three-month period
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