题目内容
Then in the early 1950's Emiliani produced the first complete record of the waxings and wanings of first glaciations. It came from a seemingly odd place, the seafloor. Single-cell marine organisms called "foraminifera" house themselves in shells made from calcium carbonate. When the foraminifera die, sink to the bottom, and become part of seafloor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen (oxygen-18) to ordinary oxygen (oxygen-16) in the carbonate preserves the ratio of the two oxygens in water molecules.
It is now understood that the ratio of oxygen isotopes reflects the proportion of the world's water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen- 18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-18 becomes in seawater--and hence in the sediments
Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earth's astronomical cycles. Since that pioneering observation, oxygenisotope measurements have been made on hundreds of cores. The combined record enables scientists to show that the re-cord contains the very periodicities as the orbital processes. Over the past 800, 000 years, the global ice volume peaked every 100,000 years, matching the period of the orbital eccentricity variation. In addition, "wrinkles" superposed on each cycle--small decreases or surges in ice volume--have come at intervals of roughly 23,000 and 41,000 years, in keeping with the precession and tilt frequencies of the Earth's spin axis.
In opening paragraph, the author introduces his topic by ______
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Li Lei didn't meet the famous American professor ______ he was on holiday in America last
As pointed out in the text, the ratio of oxygen-18 to oxygen-16 in seafloor sediments and
In the author's view, solving the cities' problem of overcrowding by strict registrations
Task 1
Directions: After reading the following passage, you will find 5 questions or unfinished statements, numbered 36 through 40. For each question or statement there are 4 choices marked A, B, C, and D. You should make the correct choice.
In many countries in the process of industrialization, overcrowded cities present a major problem. Poor conditions in these cities, such as lack of housing, inadequate sanitation(卫生) and lack of employment, bring about an increase in poverty, disease and crime.
The over-population of towns is mainly caused by the drift of large numbers of people from the rural areas. These people have become dissatisfied with the traditional life of farming, and have come to the towns hoping for better work and pay.
One possible solution to the problem would be to impose registration on town residents. Only officially registered residents would be allowed to live in the towns and the urban population would thus be limited. In practice, however, registration would be very difficult to enforce(推行); it would cause a great deal of resentment(不满), which would ultimately lead to violence.
The only long-term solution is to make life in the rural areas more attractive, which would encourage people to stay there. This could be achieved by providing encouragement for people to go and work in the villages. Facilities in the rural areas, such as transport, health and education services should be improved. Education should include training in improved methods of farming and other rural industries, so as to develop a more positive attitude towards rural life. The improvement of life in the villages is very important, because the towns themselves cannot be developed without the simultaneous or previous development of the rural area.
What does the word "urban" (Line 2, Para. 3) mean？
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