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  3. Something very unusual happened about 80,000 years ago, as Earth’s last ice age was getting started. Sea levels that had been dr

Something very unusual happened about 80,000 years ago, as Earth’s last ice age was getting started. Sea levels that had been dr

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问题     Something very unusual happened about 80,000 years ago, as Earth’s last ice age was getting started. Sea levels that had been dropping for thousands of years—as more and more water became trapped in expanding glaciers(冰川)—suddenly rose, according to a new study. Then after a few thousand years, the levels fell again. Although the researchers haven’t found the cause of this phenomenon, they say the findings could force at least a partial rethinking of the mechanisms governing Earth’s climate.
    For the past several hundred thousand years, our planet seems to have followed a fairly regular climate cycle. About every 100,000 years, kilometers-thick ice sheets form atop the northernmost reaches of North America, Europe, and Asia and extend well into the mid-latitudes. The ice sheets also tie up so much seawater that ocean levels can drop by over 100 meters. Then, after about 90,000 years, the glaciers retreat and land reappears, until the next ice age begins. The last ice age ended about 11,000 years ago.
    Various researchers going back nearly a century have attempted to isolate the cause of the ice-age cycle. They have uncovered several factors—periodic changes in Earth’s orbit around the sun, for example —that seem to be in play. But so far no one has presented the definitive answer.
    In the recently published Science, a team doesn’t so much solve the puzzle as reinforce the uncertainties about how the ice-age cycle behaves. What they’ve found is very strong evidence that when Earth should have been preparing for the last ice age, the seas rose.
    The researchers found that sea level 80,000 years ago had rebounded to the point where it rose 1 meter higher than it is today. And it could have risen quite quickly, as much as 2 meters per century, says geochemist Jeffrey Dorale of the University of Iowa.
    That suggests the glaciers were melting at a tremendous rate. Even half that rate would still be "a major finding," Dorale says. So it "has major implications for future concerns with sea-level change."
    The study "shows persuasive evidence" for a surge in sea level about 80,000 years ago, says geologist Daniel Muhs of the U.S. Geological Survey in Denver. He says the findings help make a "powerful" argument for the phenomenon. Still mysterious, however, are other data, taken from Barbados and New Guinea, that also suggest rising sea level about 80,000 years ago but not nearly as much of a change. It means, Muhs says, "that there is much about interpreting the geologic(地质学的)record of sea-level variation that we still do not understand."
What does "the puzzle"(Line 1, Para. 4)refer to?
选项 A、The cause of the ice-age cycle.
B、The duration of an ice-age cycle.
C、Whether the ice-age cycle is related to the changes in Earth’s orbit.
D、How fast the seas rose when the ice-age cycle was about to start.
答案A
解析 第4段提到的“puzzle”是针对第3段讨论的话题的，第3段讨论的主题是“冰河时代周期循环的原因”，而该段最后一句提到至今为止，没有人能找到其确切的原因，所以在第4段称之为“puzzle”，由此可见，puzzle指的是“冰河时代周期循环的原因”，因此，本题应选A。
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