You had better go at once, ____ you?
A.shouldn’t
B.wouldn’t
C.hadn’t
D.couldn’t
答案:C
伴音信号发送前采用预加重措施,接收端则采取(),以恢复原来伴音中()与()频分量振幅的比例。
下列物质中,既含有共价键又含有离子键的是
A.NaOH
B.NaCl
C.Na2O
D.HCl