单项选择题

人胚发育第6周

A.卵黄囊造血
B.肝脏造血
C.脾脏造血
D.胸腺和淋巴结造血
E.骨髓造血
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A Sunshade for the Planet?? Evenwith the best will in the world,reducing our carbon emissions is not going toprevent global warming. It has become clear that even if we take the moststrong measures to control emissions, the uncertainties in our climate modelsstill leave open the possibility of extreme warming and rises in sea level. Atthe same time,resistance by governments and special interest groups makes itquite possible that the actions suggested by climate scientists might not beimplemented soon enough.???? Fortunately,if the worst comes to the worst,scientists still have a few tricks up their sleeves. For the most part theyhave strongly resisted discussing these options for fear of inviting a sense ofcom- placency that might thwart efforts to tackle the root of the problem.Until now, that is. A growing number of researchers are taking a fresh look atlarge-scale geo-engineering projects that might be used tocounteract global warming. I use the analogy of methadone, saysStephen Schneider,a cli- mate researcher at StanfordUniversity in California who was among the first to drawattention to global warming. If you have a heroin addict, the correcttreatment is hospitalization,and a long rehaB.But if they absolutelyrefuse,methadone is better than heroin. ???? Basically the idea is to apply sunscreen to the wholeplanet. One astronomer has come up with a radical plan to cool Earth;launchtrillions of feather-light discs into space, where they would form. a vast cloudthat would block the suns rays. Its controversial,but recent studies suggestthere are ways to deflect just enough of the sunlight reaching the Earthssurface to counteract the warming produced by the greenhouse effect. Globalclimate models show that blocking just l. 8 per cent of the incident energy inthe suns rays would cancel out the warming effects produced by a doubling ofgreenhouse gases in the atmospherE.That could be crucial,because even the mostsevere emissions-control meas- ures being proposed would leave us with a doublingof carbon dioxide by the end of this century,and that would last for at least acentury morE.????According to the first two paragraphs, theauthor thinks that _________?A.strong measures have been taken by the government to prevent global warmingB.to reduce carbon emissions is all impossible missionC.despite the difficulty, scientists have some options to prevent global warmingD.actions suggested by scientists will never be realized
A.??
??
B.Until
C.??
??According
D.strong
E.to
F.despite
G.actions
单项选择题
辅阻遏物一般是操纵子结构基因表达的酶所催化反应的( )。 A.底物 B.终产物 C.诱导物 D.阻遏蛋白
A.底物
B.终产物
C.诱导物
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