2. 外语
  3. In fact, even without humans, the Earth’s climate changes. Some climate change is【C1】______. But, as greenhouse gases are added

In fact, even without humans, the Earth’s climate changes. Some climate change is【C1】______. But, as greenhouse gases are added

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问题     In fact, even without humans, the Earth’s climate changes. Some climate change is【C1】______. But, as greenhouse gases are added to the atmosphere, human influence "emerges" from natural variability.
    Droughts, one of the most intensely studied climate events, are a perfect example of a(n) 【C2】______with both natural and human influences. Separating the【C3】______strengths of the influences is a challenge for scientists. However, with the large social and economic costs of droughts, it is a challenge the scientists must【C4】______.
    In a very recent study published in the Journal of Climate, authors Richard Seager and Martin Hoerling cleverly used climate models forced by sea surface temperatures to【C5】______how much of the past century’s North American droughts have been caused by ocean temperatures, natural variability and human influences.
    Droughts can be caused by a(n) 【C6】______of separate or interactional phenomena. At its root, drought results from the low【C7】______of water falling and sometimes higher temperatures(which increase evaporation rates). The beginning of drought can often be linked to variations in ocean temperatures.
    It is also found that the oceans can affect the atmosphere to create conditions that are【C8】______responsible for drought. What’s more, temperature increases【C9】______with human-driven global warming also play a role. This【C10】______agrees with other researchers who have shown that, while human-emitted greenhouse gas warming may not cause a particular drought, it can make drought come on earlier, faster, and harder than it otherwise would.
A)associated
B)attached
C)conclusion
D)conduct
E)distinguish
F)effect
G)natural
H)partly
I)quality
J)quantity
K)relative
L)ridiculous
M)simply
N)undertake
O)variety
【C4】
选项
答案N
解析 空格位于名词challenge后面的定语从句中，而且与情态动词must共同作谓语，故应填入动词原形。上文提到，对于科学家来说，区分自然因素和人为因素对干旱所造成影响的相对强度是一个挑战。但是，由于干旱造成的巨大的社会和经济损失，科学家们必须承担这一挑战，故空格处应填入un—dertake“承担”。备选的动词中，conduct“进行，实施”和distinguish“区别”均不符合文意，故排除。
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大学英语四级

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