Which two statements are true about broadband cable （HFC） systems？（）
A. Cable modems only operate at Layer 1 of the OSI model.
B. Cable modems operate at Layers 1 and 2 of the OSI model.
Cable modems operate at Layers 1， 2， and 3 of the OSI model.
D. A function of the cable modem termination system （CMTS） is to convert the modulated signal from the cable modem into a digital signal.
E. A function of the cable modem termination system is to convert the digital data stream from the end user host into a modulated RF signal for transmission onto the cable system.
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