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  3. The planet is getting lusher, and we are responsible. Carbon dioxide generated by human activity is stimulating photosynthesis (

The planet is getting lusher, and we are responsible. Carbon dioxide generated by human activity is stimulating photosynthesis (

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问题    The planet is getting lusher, and we are responsible. Carbon dioxide generated by human activity is stimulating photosynthesis (光合作用) and causing a beneficial greening of the Earth’s surface. For the first time, researchers claim to have shown that the increase in plant cover is due to this "CO2 fertilization effect" rather than other causes. However, it remains unclear whether the effect can counter any negative consequences of global warming, such as the spread of deserts.
   Recent satellite studies have shown that the planet is harboring more vegetation overall, but pinning down the cause has been difficult. Factors such as higher temperatures, extra rainfall, and an increase in atmospheric CO2—which helps plants use water more efficiently—could all be boosting vegetation. To home in on the effect of CO2, Randall Donohue of Australia’s national research institute, the CSIRO in Canberra, monitored vegetation at the edges of deserts in Australia, southern Africa, the US Southwest, North Africa, the Middle East and central Asia. These are regions where there is ample warmth and sunlight, but only just enough rainfall for vegetation to grow, so any change in plant cover must be the result of a change in rainfall patterns or CO2levels, or both.
   If CO2 levels were constant, then the amount of vegetation per unit of rainfall ought to be constant, too. However, the team found that this figure rose by 11 per cent in these areas between 1982 and 2010, mirroring the rise in CO2( Geophysical Research Letters, doi. org/mqx). Donohue says this lends "strong support" to the idea that CO2 fertilization drove the greening.
   Climate change studies have predicted that many dry areas will get drier and that some deserts will expand. Donohue’s findings make this less certain. However, the greening effect may not apply to the world’s driest regions. Beth Newingham of the University of Idaho, Moscow, recently published the result of a 10-year experiment involving a greenhouse set up in the Mojave Desert of Nevada. She found "no sustained increase in biomass" when extra CO2 was pumped into the greenhouse. "You cannot assume that all these deserts respond the same," she says. "Enough water needs to be present for the plants to respond at all."
Which of the following is the explanation of " Recent satellite studies have shown that the planet is harboring more vegetation overall, but pinning down the cause has been difficult"(Para. 2)?
选项 A、Our planet hides more vegetation, but dealing with these issues is very difficult.
B、Our planet lacks of planets, but the reason why there are so many planets is difficult to be identified.
C、Vegetation will be more and more in our planet, but it is difficult to find the cause.
D、More plants hide in our planet, which are not discovered by people, but determining the reason is very difficult.
答案D
解析 根据题干可以将该题定位至文章第二段。题目要求是对句子Recent satellite studies have shown that the planet is harboring more vegetation overall，but pinning down the cause has been difficult的解释。根据选项，主要是要判断harboring和pinning down的意思。harbor的本意是“港湾”，其主要功能是让船能躲避风浪，在这里意为“隐匿，隐藏”；pinning down意为“确定”。因此答案为D选项。
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