【单选题】

______in the strength of the carrier signal is varied to represent binary 1 or 0．

______is a system that can map a name to an address and conversely an address to an name．
______is a three-way handshaking authentication protocol that it’s packets are encapsulated in the point-to-point protocol
______is an approach that enables anATM Switch to behave like a LAN switch．
______is an guided media which is made of glass or plastic and transmits signals in the form of light．
ASK B．PSK
C、FSK
D、FDM