Which of the following statements can best describe the telephone in the U. S.?
A.It's very inexpensive.
B.It saves a lot of trouble.
C.It's very convenient.
D.A, B and
第5题
用肥皂水(发泡剂)检漏液适用于()气体管道的泄漏,不适用于真空管道。
A.正压
B.负压
C.低压
D.高压
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