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问题 单项选择题

将大肠杆菌放在15N培养基中培养若干代后，大肠杆菌DNA中所有的氮均为15N。然后将被15N标记的大肠杆菌DNA，转放到14N培养基中，连续培养三代，在第三代大肠杆菌DNA总量中带有15N标记的DNA占（）

A.12.5%

B.25%

C.5%

D.75%

答案

参考答案：B

问答题

Passage 2 The Truth about the Environment For many environmentalists, the world seems to be getting worse. They have developed a hit-list of our main fears: that natural resources are running out; that the population is ever growing, leaving less and less to eat; that species are becoming extinct in vast numbers, and that the planet’s air and water are becoming ever more polluted. But a quick look at the facts shows a different picture. First, energy and other natural resources have become more abundant, not less so, since the book The Limits to Growth was published in 1972 by a group of scientists. Second, more food is now produced per head of the world’s population than at any time in history. Fewer people are starving. Third, although species are indeed becoming extinct, only about 0.7% of them are expected to disappear in the next 50 years, not 25-50%, as has so often been predicted. And finally, most forms of environmental pollution either appear to have been exaggerated, or are temporary-associated with the early phases of industrialisation and therefore best cured not by restricting economic growth, but by accelerating it. One form of pollution—the release of greenhouse gases that causes global warming—does appear to be a phenomenon that is going to extend well into our future, but its total impact is unlikely to pose a devastating problem. Yet opinion polls suggest that many people nurture the belief that environmental standards are declining and many factors seem to cause this disjunction between perception and reality.
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单项选择题

已知2cm厚垂直式一般封闭空气间层的热阻为Rg((m2·K)／W)，下列封闭空气问层热阻R的表示式，哪一项不正确( )

A．热流向上的2cm厚水平式一般封闭空气间层，R

B．热流向下的2cm厚水平式一般封闭空气问层，R>Rg

C．4cm厚垂直式一般封闭空气间层，R=2Rg

D．热流向下4cm厚水平式封闭空气间层，R<2Rg
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