题目内容
计算机网络体系结构是一种层次结构，OSI/RM的传输（第4）层处于网络层提供的服务之上，给会话层提供服务
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Loudspeaker is a transducer which converts the state of the controlled quantity to an electrical signal.
Audio amplifier can increase the strength(power) of the audio signal.
Power amplifier can decrease the strngth(power) of th udio signal.
Microphone is a transducer which converts sound to voltage.
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