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  3. When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, t

When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, t

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问题     When the first of the two Viking landers touched down on Mars on July 20, 1976, and began to send camera images back to earth, the scientists at the Jet Propulsion Laboratory could not suppress a certain nervous anticipation, like people holding a lottery ticket that they have a one-in-a-million chance of winning. The first photographs that arrived, however, did not contain any evidence of life. What revealed itself to them was merely a barren landscape littered with rocks and boulders. The view resembled nothing so much as a flat section of desert.
    The scientists were soon ready to turn their attention from visible life to microorganisms. The twin Viking landers carried three experiments designed to detect current biological activity and one to detect organic compounds, because researchers thought it possible that life had developed on early Mars just as it is thought to have developed on earth, through the gradual chemical evolution of complex organic molecules. To detect biological activity, Martian soil samples were treated with various nutrients that would produce characteristic by-products if life forms were active in the soil. The results from all three experiments were inconclusive. The fourth experiment heated a soil sample to look for signs of organic material, but found none, an unexpected result because at least organic compounds from the bombardment of the Martian surface by meteorites were thought to have been present.
    The absence of organic materials, some scientists speculated, was the result of intense ultraviolet radiation penetrating the atmosphere of Mars and destroying organic compounds in the soil. Although Mars’ atmosphere was, at one time, rich in carbon dioxide and thus thick enough to protect its surface from the harmful rays of the sun, the carbon dioxide had gradually left the atmosphere and been converted into rocks. This means that even if life had gotten a start on early Mars, it could not have survived the exposure to ultraviolet radiation when the atmosphere thinned.
    Despite the disappointing Viking results, there are those who still keep the possibility of life on Mars open. They point out that the Viking data cannot be considered the final word on Martian life because the two landers only sampled two limited—and uninteresting—sites. The Viking landing sites were not chosen for what they might tell of the planet’s biology. They were chosen primarily because they appeared to be safe for landing a spacecraft. The landing sites were on parts of the Martian plains that appeared relatively featureless from orbital photographs.
    The type of Martian terrain that these researchers suggest may be a possible hiding place because active life has an earthly parallel: the ice-free region of southern Victoria Land, Antarctica, where the temperatures in some dry valleys average below zero. Organisms known as endoliths, a form of blue-green algae that has adapted to this harsh environment, were found living inside certain rocks in these Antarctic valleys. The argument based on this discovery is that if life did exist on early Mars, it is possible that it escaped worsening conditions by similarly seeking refuge in rocks. Skeptics object, however, that Mars in its present state is simply too dry, even compared with Antarctic valleys, to sustain any life whatsoever.
    Should Mars eventually prove to be completely barren of life, as some suspect, then this would have a significant impact on the current view of the chemical origin of life. It could be much more difficult to get life started on a planet than scientists thought before the Viking landings.
What did the author emphasize by mentioning the Viking landing sites in Paragraph 4?
选项 A、Uninteresting as the landing sites were, they could have harbored Martian life.
B、The landers’ failure of finding evidence of life does not mean that Mars is devoid of life.
C、The Viking mission was unsuccessful largely due to selection of improper landing sites.
D、Scientists were not expecting to discover life on the Martian plains.
答案B
解析 第4段第2句提到，维京号的实验数据并不能对“火星上是否存在生命”这一讨论一锤定音，因为登陆地点的选择相对局限且没有特点，所以B项“探测器没找到生命迹象的证据不意味着火星上不存在生命”，正是作者着重提到着陆地点的原因。A项“登陆地点虽然普通，它们还是可能蕴藏火星生命”，这与文意恰好相反，因此排除；C项“维京号任务的失败主要因为登陆地点选择不恰当”，文中提到登陆地点的选择是出于安全考虑，因此C项也排除；D项“科学家没有期待能从火星平原上发现生命”，与文意不符，排除。
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