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【单选题】
点燃氢气前必须检验氢气的纯度，是因为（ ）A.
氢气有可燃性
B.
氢气有还原性
C.
氢气比空气轻
D.
氢气中混有空气时，点燃会发生爆炸
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参考解析：
知识点：
皮皮学刷刷变学霸
举一反三
【单选题】I wanted to have him ______ the job, but she had it _____ already.
A.
do, done
B.
does, doing
C.
to do, doing
D.
to do, done
【单选题】I wanted to have him _____ the job, but he had it _____ instead.
A.
do; done
B.
does; be done
C.
to do; doing
D.
to do; done
【简答题】对于后续专业课程的学习,你有什么想要重点了解的内容?
【单选题】I wanted to have him _____ the job, but she had it _____ instead.
A.
do/done
B.
does/doing
C.
to do/doing
【多选题】连锁经营物流集中配送系统包括( )。
A.
TC（transfer center）系统
B.
DC（distribution center）系统
C.
PC（process center）系统
【单选题】I wanted to have him _____ the job, but she had it _____ instead.
A.
do; done
B.
does; doing
C.
to do; doing
D.
to do; done
【简答题】I wanted to have him
_____ the job, but she had it _____ instead.A. do/done B. does/doing C. to
do/doing D. to do/done
【多选题】How is recombinant DNA cloned in bacteria?
A.
To clone a particular fragment of DNA, it is first inserted into a plasmid vector. To insert a piece of DNA into a plasmid vector, the purified plasmid DNA is opened up by a restriction enzyme that cleaves it at a single site, and the DNA fragment to be cloned is then spliced into that site using DNA ligase. This recombinant DNA molecule is now ready to be introduced into a bacterium, where it will be copied and amplified.
B.
The resulting recombinant plasmid DNA is then introduced into a bacterium by transformation. These bacteria are then suspended in a nutrient-rich broth and allowed to proliferate.
C.
In 24 hours, the engineered cells will produce hundreds of millions of copies of the plasmid, along with the DNA fragment it contains. The bacteria can then be split open (lysed) and the plasmid DNA purified from the rest of the cell contents, including the large bacterial chromosome.
D.
The DNA fragment can be readily recovered by cutting it out of the plasmid DNA with the same restriction enzyme that was used to insert it, and then separating it from the plasmid DNA by gel electrophoresis. Together, these steps allow the amplification and purification of any segment of DNA from the genome of any organism.
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