阅读理解 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.

单选题 21.We can learn from the first paragraph that______.

【正确答案】 C

【答案解析】根据选项[A]中的Mauna Loa定位到首句：．．．the Manna Loa Observatory in Hawaii…说明Mauna Loa是在夏威夷而不是加拿大。根据选项[B]定位到尾句：．．．4m years ago，when jungles covered northern Canada．故[B]项错误。[C]项对应原文：．．．researchers at the observatory expect this figure，too，to exceed 400 in the next few days．而this figure指代的就是carbon—dioxide concentration，故正确。[D]项对应原文：The last time such values prevailed on Earthwas in the Pliocene epoch 4m years ago…说明达到400 ppm这个数据不是历史第一次。综上所述，该题答案为选项[C]。

单选题 22.Mauna Loa is the standard for CO₂ measurement for the reason that______.

【正确答案】 A

【答案解析】根据CO₂ measurement定位到第二段第二行：Mauna Loa is the benchmark for CO₂measurement because Hawaii is so far from large concentrations of humanity．题干问的是原因，forthe reason that=because，故答案在because之后，而Hawaii is so far from large concentrations ofhumanity=Hawaii has small concentrations of human，故[A]项为答案。

单选题 23.The third paragraph tells us that______.

【正确答案】 D

【答案解析】首先[A]和[B]是正话反说，其实表达的是同一个意思，可以排除。这两项所对应的原文为：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．说明CO₂浓度是五月到十月下降，即夏天下降，而冬天到春天期间上涨，[A]和[B]两项和原文意思完全相反。选项[C]对应原文:．．．figure will reach 400 ppm in the spring of 2014 or 2015．故选项[C]错误。选项[D]对应原文：Pieter Tans，who is in charge of monitoring at Mauna Loa…其中monitoring=supervision。故答案为选项[D]。

单选题 24.The rise of green house gas emissions leads to______.

【正确答案】 B

【答案解析】原文第四段最后一句为：The rate of increase is now 2．1 ppm--three times as fast—reflecting the relentless rise in green—house—gas emissions．意为：增长率如今是2．1 ppm——是之前的三倍——反映了温室气体排放量不断增长。也就是说温室气体排放导致了ppm增长率变快，即选项[B]是正确的。

单选题 25.Which is true according to the last paragraph?

【正确答案】 A

【答案解析】[A]项对应原文：Because CO₂ stays in the atmosphere for decades…其中stays=re—mains；decades=several dozen years，故选项[A]正确。[B]项对应原文：At current rates，theMauna Loa reading will rise above 450 ppm in 2037．原文提到的是2037年，而非2075年，故选项[B]错误。[C]项对应原文．．．．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．原文说的是no chance，该选项说的是chance is good，故错误。[D]项也对应以上原文。原文说的是have to be cut，而不是have been cut，故选项[D]错误。答案为选项[A]。