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 al 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.

A．NAT

B．NCP

C．DDN

D．DNS

目前使用疫苗中既有减毒活疫苗又有灭活疫苗的是（）。

A、乙脑疫苗

B、乙肝疫苗

C、脊灰疫苗

D、甲肝疫苗