TPO19 閱讀第三篇Discovering the Ice Ages細節題 - 英檢

不好意思又來請教各位
因為TPO 19有些版友可能沒有, 所以附上全文..
如果美加助教可以幫我轉問老師, 我真的會非常的感激..

Discovering the Ice Ages

In the middle of the nineteenth century, Louis Agassiz, one of the first
scientists to study glaciers, immigrated to the United States from Switzerland
and became a professor at Harvard University, where he continued his studies
in geology and other sciences. For his research, Agassiz visited many places
in the northern parts of Europe and North America, from the mountains of
Scandinavia and New England to the rolling hills of the American Midwest.
In all these diverse regions, Agassiz saw signs of glacial erosion and
sedimentation. In flat plains country, he saw moraines (accumulations of
earth and loose rock that form at the edges of glaciers) that reminded him
of the terminal moraines found at the end of valley glaciers in the Alps.
The heterogeneous material of the drift (sand, clay, and rocks deposited
there) convinced him of its glacial origin.

◆ The areas covered by this material were so vast that the ice that deposited
it must have been a continental glacier larger than Greenland or Antarctica.
Eventually, Agassiz and others convinced geologists and the general public
that a great continental glaciation had extended the polar ice caps far into
regions that now enjoy temperate climates. For the first time, people began to
talk about ice ages. It was also apparent that the glaciation occurred in the
relatively recent past because the drift was soft, like freshly deposited
sediment. We now know the age of the glaciation accurately from radiometric
dating of the carbon-14 in logs buried in the drift. The drift of the last
glaciation was deposited during one of the most recent epochs of geologic time,
the Pleistocene, which lasted from 1.8 million to 10,000 years ago. Along the
east coast of the United States, the southernmost advance of this ice is
recorded by the enormous sand and drift deposits of the terminal moraines that
form Long Island and Cape Cod.


19: It can be inffered from paragraph 2 that Agassiz and other geolosists of
his time were not able to determin


A. which geographic regions had been covered with ice sheets in the lat ice age
B. the exact dates at which drifts had been deposited during the last ice age
C. the exact composition of the drifts laid during the last ice age

答案是B.
想請教說文中不是已經說we now know the age of the glaciation accurately from...
答案怎麼會是B呢!? A與C反而還比較像答案不是嗎??

It soon became clear that there were multiple glacial ages during the
Pleistocene, with warmer interglacial intervals between them. As geologists
mapped glacial deposits in the late nineteenth century, they became aware
that there were several layers of drift, the lower ones corresponding to
earlier ice ages. Between the older layers of glacial material were
well-developed soils containing fossils of warm-climate plants. These soils
were evidence that the glaciers retreated as the climate warmed. By the early
part of the twentieth century, scientists believed that four distinct
glaciations had affected North America and Europe during the Pleistocene epoch.
This idea was modified in the late twentieth century, when geologists and
oceanographers examining oceanic sediment found fossil evidence of warming and
cooling of the oceans. Ocean sediments presented a much more complete geologic
record of the Pleistocene than continental glacial deposits did. The fossils
buried in Pleistocene and earlier ocean sediments were of foraminifera—small,
single-celled marine organisms that secrete shells of calcium carbonate, or
calcite. These shells differ in their proportion of ordinary oxygen (oxygen-16)
and the heavy oxygen isotope (oxygen-18). The ratio of oxygen-16 to oxygen-18
found in the calcite of a foraminifer's shell depends on the temperature of
the water in which the organism lived. Different ratios in the shells preserved
in various layers of sediment reveal the temperature changes in the oceans
during the Pleistocene epoch.

Isotopic analysis of shells allowed geologists to measure another glacial
effect. They could trace the growth and shrinkage of continental glaciers,
even in parts of the ocean where there may have been no great change in
temperature—around the equator, for example. "The oxygen isotope ratio of the
ocean changes as a great deal of water is withdrawn from it by evaporation and
is precipitated as snow to form glacial ice." During glaciations, the lighter
oxygen-16 has a greater tendency to evaporate from the ocean surface than the
heavier oxygen-18 does. Thus, more of the heavy isotope is left behind in the
ocean and absorbed by marine organisms. From this analysis of marine sediments,
geologists have learned that there were many shorter, more regular cycles of
glaciation and deglaciation than geologists had recognized from the glacial
drift of the continents alone.

25: In paragraph 5, why does the author include the information that the
"The oxygen isotope ratio of the ocean changes as a great deal of water
is withdrawn from it by evaporation and is precipitated as snow to form
glacial ice."??

答案:
A. To explain how scientists were able to calculate how frequently the
continental ice sheets expanded and contracted.

請問這答案在文中哪邊找的到呀? 完全不了解中...

麻煩各位解答了, 謝謝您花時間解答我的疑惑..

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