某品牌早餐奶的包装中标注的营养成分如图所示，标注中的“铁”和_爱下问搜题

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问题选择题

某品牌早餐奶的包装中标注的营养成分如图所示，标注中的“铁”和“锌”是指（　　）

A．单质

B．分子

C．元素

D．原子

答案

A、铁、锌的单质是金属，而奶中不存在金属铁，可通过磁铁吸引法验证，所以错误．

B、铁、锌不存在分子，它们只能由原子直接构成金属单质，或形成化合物，所以错误．

C、铁、锌等一般以无机盐的形式存在于奶中，是组成无机盐的基本成分，因此“铁、锌”应理解为元素，所以正确．

D、铁、锌原子直接构成金属单质，而奶中不存在金属铁，可通过磁铁吸引法验证，所以错误．

故选C．

问答题简答题

PET如何进行心肌存活的测定?

单项选择题

Networks can be interconnected by different devices in the physical layer networks can be connected by (71) 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 (72) ，examine the MAC address and forward the frames to a different network while doing minor protocol translation in the process in me network layer，we have routers that can connect two networks，If two networks have (73) network layer，the router may be able to translate betweenthe 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 (74) ．As an example，gateways between Internet e-mail and X，400 e-mail must (75) thee-mail message and change various header fields．

A．syntax

B．semantics

C．language

D．format