When a digital signal is transmitted in an analog transmission system, what are required at the transmitting and the receiving ends respectively? A: Modulator and Demodulator B: Demodulator and Modulator C: Encoder and decoder D: Decoder and encoder

（）是Modulator(调制器)与Demodulator(解调器)的简称，中文称为调制解调器(港台称之为数据机)，根据Modem的谐音，亲昵地称之为"（）"。

. Which function should not be performed by the holder of a GMDSS operator's license on a NAVTEX receiver? A: Replace printer paper B: Correcting paper jamming C: . Repair demodulator D: Self-diagnostic printer and receiver tests

调制解调器由调制器(MOdulator)和解调器(DEModulator)两部分组成，其中解调器实现的功能是（ ） A: 把模拟信号转变为数字信号 B: 把数字信号转变为模拟信号 C: 把模拟信号转变为音频信号 D: 把音频信号转变为模拟信号