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.
• C、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.