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问题 问答题

在某公共场所张贴了如下广告，请计算并回答下列问题：

优质化肥  国内首创

尿素[CO（NH2）2]

含氮量全球最高（48%）

（1）尿素中各元素的质量比；

（2）尿素中氮元素的质量分数；

（3）这一则广告是真实的吗？

答案

（1）尿素中C、O、N、H元素的质量比为12：16：（14×2）：（1×4）=3：4：7：1．

（2）尿素中氮元素的质量分数为=

14×2
60
×100%=46.7%．

（3）∵尿素中氮元素的质量分数46.7%＜48%，

∴这则广告不真实．

故答案为：（1）3：4：7：1；（2）46.7%；（3）这则广告不真实．

单项选择题

Border Gateway Protocol(BGP) is inter-autonomous system （71） protocol. BGP is based on a routing method called path vector routing. Distance vector routing is not a good candidate for inter-autonomous system routing because there are occasions on which the route with the smallest （72） count is not the preferred route. For example, we may not want a packet through an autonomous system that is not secure even though it is shortest route. Also, distance vector routing is unstable due to the fact that the routers announce only the number of hop counts to the destination without defining the path that leads to that （73） . A router that receives a distance vector advertisement packet may be fooled if the shortest path is actually calculated through the receiving router itself. Link （74） routing is also not a good candidate for inner-autonomous system routing because an internet is usually too big for this routing method. To use link state routing for the whole internet would require each router to have a huge link state database. It would also take a long time for each router to calculate its routing （75） using the Dijkstra algorism.

75（）

A．table

B．state

C．metric

D．cost
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单项选择题

土的强度在冻结时大大提高，冻土的抗压强度比抗拉强度大2~3倍。不可选择的开挖方法是（）

A.人工法

B.铲运机法

C.机械法

D.爆破法
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