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问题 填空题

已知A（2，-4），B（2，-6），那么线段AB=______．

答案

根据平面内两点之间的距离公式为：AB=

(2-2) 2+(-4+6) 2
=2，

也可根据A，B两点横坐标相等，距离就是纵坐标之间的距离：-4+6=2．

故答案为：2．

单项选择题

能与林可霉素竞争同一结合部位是拮抗作用的药物是（）

A.链霉素

B.万古霉素

C.青霉素

D.红霉素

E.替考拉宁
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单项选择题

Networks can be interconnected by different devices. In the physical layer, networks can be connected by (1) or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept (2) , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have (3) network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message (4) . As an example, gateways between Internet e-mail and X.400 e-mail must (5) the e-mail message and change various header fields.

A．syntax

B．semantics

C．language

D．format
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