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  3. At noon on May 4th the carbon-dioxide concentration in the atmosphere around the Mauna Loa Observatory in Hawaii hit 400 parts p

At noon on May 4th the carbon-dioxide concentration in the atmosphere around the Mauna Loa Observatory in Hawaii hit 400 parts p

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问题     At noon on May 4th the carbon-dioxide concentration in the atmosphere around the Mauna Loa Observatory in Hawaii hit 400 parts per million (ppm). The average for the day was 399.73 and researchers at the observatory expect this figure, too, to exceed 400 in the next few days. The last time such values prevailed on Earth was in the Pliocene epoch (上新世) 4m years ago, when jungles covered northern Canada.
    There have already been a few readings above 400 ppm elsewhere—those taken over the Arctic Ocean in May 2012, for example—but they were exceptional. Mauna Loa is the benchmark (标准) for CO2 measurement because Hawaii is so far from large concentrations of humanity. The Arctic, by contrast, gets a lot of polluted air from Europe and North America.
    The concentration of CO2 peaks in May, falls until October as plant growth in the northern hemisphere’s summer absorbs the gas, and then goes up again during winter and spring. This year the average reading for the whole month will probably also reach 400 ppm, according to Pieter Tans, who is in charge of monitoring at Mauna Loa, and the seasonally adjusted annual figure will reach 400 ppm in the spring of 2014 or 2015.
    Mauna Loa’s readings are one of the world’s longest-running measurement series. The first, made in March 1958, was 315 ppm. That means they have risen by a quarter in 55 years. In the early 1960s they were going up by 0.7 ppm a year. The rate of increase is now 2.1 ppm—three times as fast—reflecting the relentless rise in green-house-gas emissions.
    As a rule of thumb, CO2 concentrations will have to be restricted to about 450 ppm if global warming is to be kept below 2 degrees. Because CO2 stays in the atmosphere for decades, artificial emissions of the gas would have to be cut immediately, and then fall to zero by 2075, in order to achieve 450 ppm. There seems no chance of that. Emissions are still going up. At current rates, the Mauna Loa reading will rise above 450 ppm in 2037.
Mauna Loa is the standard for CO2 measurement for the reason that ______.
选项 A、Hawaii has small concentrations of human
B、its air has been polluted by Europe and North America
C、Hawaii has far larger concentrations of humanity than other places
D、The Arctic gets a lot of polluted air from Europe and North America
答案A
解析 根据题干中的“CO2 measurement”定位到第二段第二行：Mauna Loa is the bench-mark for CO2 measurement because Hawaii is so far from large concentrations of humanity. 题干问的是原因，“for the reason that”=“because”，故答案在because之后，而“Hawaii is so far from large concentrations of humanity”=“Hawaii has small concentrations of human”，故A项为答案。
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