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问题 单项选择题

培养基的冷却方式有喷淋冷却式、真空冷却式、薄板换热器式几种方式，其中（）效率高，且利用冷培养基作冷却剂，既冷却了热培养基，又预热了冷培养基，节约用水和蒸汽。

A、喷淋冷却式

B、真空冷却式

C、薄板换热器式

D、A+B+C

答案

参考答案：C

单项选择题

Networks can be interconnected by different devices. In the physical layer, networks can be connected by (11) 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 (12) ,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 (13) network layer, the router may be able to translate between the packer 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 (14) . As an example, gateways between Internet e-mail and X.400 e-mail must (15) the e-mail message and change various header fields.

12（）

A.frames

B.packets

C.packages

D.cells
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填空题

柑桔黄龙病的传播途径是（）
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