(1) In the strength of the carrier signal
题型：单项选择题
问题：
(1) In the strength of the carrier signal is varied to represent binary 1 or 0.
(2) is a system that can map a name to an address and conversely an address to an name.
(3) is a three-way handshaking authentication protocol that it’s packets are encapsulated in the point-to-point protocol
(4) is an approach that enables an ATM Switch to behave like a LAN switch.
(5) is an guided media which is made of glass or plastic and transmits signals in the form of light.
2（）
A．NAT
B．NCP
C．DDN
D．DNS