【单选题】

(71) In the strength of the carrier signal is varied to represent binary 1 or 0.

(72) is a system that can map a name to an address and conversely an address to an name.
(73) is a three-way handshaking authentication protocol that it’s packets are encapsulated in the point-to-point protocol
(74) is an approach that enables anATM Switch to behave like a LAN switch.
(75) is an guided media which is made of glass or plastic and transmits signals in the form of light.
A．PPP
B．PAP
CHAP
D．LCP