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问题 选择题

下列实例中能体现细胞全能性的是

①用胡萝卜单个细胞培养成可育的植株 ②植物用种子进行繁殖 ③用烟草的单个组织细胞培养成可育的植株[ ]

A．①②

B．①③

C．②③

D．①②③

答案

答案：B

选择题

济南的白莲藕品质优良，荷花又是济南的市花。它通常是如何繁殖的？（   ）

A．用果实繁殖

B．用种子繁殖

C．用茎繁殖

D．用根繁殖
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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 (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.

11（）

A.repeaters

B.relays

C.packages

D.modems
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