单选题 For each blank, choose the best answer from the four choices and write down on the answer sheet.
In (66) the strength of the carrier signal is varied to represent binary 1 or 0. (67) is a system that can map a name to an address and conversely an address to an name. (68) is a three -way handshaking authentication protocol that it's packets are encapsulated in the point - to -point protocol. (69) is an approach that enables an ATM Switch to behave like a LAN switch. (70) is an guided media which is made of glass or plastic and transmits signals in the form of light.