单选题Eight times within the past million years, something in the Earth"s climatic equation has changed, allowing snow in the mountains and the northern latitudes to accumulate from one season to the next instead of melting away. Each time, the enormous ice sheets resulting from this continual buildup lasted tens of thousands of years until the end of each particular glacial cycle brought a warmer climate. Scientists speculated that these glacial cycles were ultimately driven by astronomical factor: slow, cyclic changes in the eccentricity of the Earth"s orbit and in the tilt and orientation of its spin axis. But up until around 30 years ago, the lack of an independent record of ice-age timing made the hypothesis untestable. 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.

单选题______ the surface of metal, but also weakens it.

单选题Because of their out-of-date organization some unions find it difficult to

单选题Although she was still ill, she ______ herself from the hospital. A. extracted B. discharged C. injected D. drained

单选题Prof. White, my respected tutor, frequently reminds me to ______ myself of every chance to improve my English.

单选题Mail service will be temporarily ______ because of the strike of the postal workers.

单选题根据下面资料，回答21-35题。Mrs.McTavish looked out."It's a lovely day.Would you like to go for a walk in the park？"Her children 21 with excitement. "Before we go， you need to follow some rules.Everyone must 22 h

单选题The students were told to make ______ study of the air situation in ______ city of Beijing.

单选题A: Your little boy has done a good job at school.B: ______

单选题Her jewelry______under the spotlights and she became the dominant figure at the ball.

单选题Man:I'm really exhausted．But I don't want to miss the film that comes on at 11. Woman:If I were you, I'd skip it. We both have to get up early tomorrow. And anyway, I've heard it isn't that exciting. Question:What does the woman mean?

单选题Which of the following is TRUE about the hours of the library?

单选题Automation threatens mankind with an increased number of ______ hours. A. useless B. active C. complex D. idle

单选题I would like to apply ______ the position of sales clerk that you advertised in the Sunday news- paper.

单选题The teacher, with 6 girls and 8 boys of her class,______ visiting a museum when the earthquake struck.

单选题The private life of having each individual make his or her own choice of beliefs and interest ______ without the overarching public world of the state, which sustains a structure of law appropriate to a self-determining association.

单选题There is not much news in today's paper, ______?

单选题If the boy had ______ the dog alone it wouldn't have bitten him. A. set B. left C. had D. put

单选题The company used so much coal that it has a train to ______ the delivery process. A. transmit B. transport C. facilitate D. diminish

单选题He is sincere and easy to ______.