______in the strength of the carrier signa

问题：

______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 an ATM 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．

A．NAT 

B．NCP 

C．DDN

D．DNS