Which statement about CSU/DSU is true:

A、Achannelserviceunit/dataserviceunit(CSU/DSU)isrequiredwhendigitalequipmentwillbeusedtoconnectaLANtoaWAN.
B、ACSU/DSUisnecessarybecausethesignalsandframescanbesamebetweentheLANequipmentandtheWANequipmentusedbyserviceproviders.
C、TheDSUdeviceconvertsdigitalsignalsfromrouters,switches,andmultiplexersintosignalsthatcanbetransmittedoverthedatareceiverlines.
D、TheDSUdeviceensuresthatthevoltagelevelsarecorrect,theinformationcanbelostduringtheconversion.