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搜尋:plankton

Questions 31-39 Perhaps one of the most dramatic and important changes that took place in the Mesozoic era occurred late in that era, among the small organisms that populate the uppermost, sunlit portion of the oceans--the plankton. The term "plankton" is a broad Line one, designating all of the small plants and animals that float about or weakly propel (5) themselves through the sea. In the late stages of the Mesozoic era. during the Cretaceous period, there was a great expansion of plankton that precipitated skeletons or shells composed of two types of mineral: silica and calcium carbonate. This development radically changed the types of sediments that accumulated on the seafloor, because, while the organic parts of the plankton decayed after the organisms died, their mineralized (10) skeletons often survived and sank to the bottom. For the first time in the Earth's long history, very large quantities of silica skeletons, which would eventually harden into rock, began to pile up in parts of the deep sea. Thick deposits of calcareous ooze made up of the tiny remains of the calcium carbonate-secreting plankton also accumulated as never before. The famous white chalk cliffs of Dover, in the southeast of England, are just one (15) example of the huge quantities of such material that amassed during the Cretaceous period; there are many more. Just why the calcareous plankton were so prolific during the latter part of the Cretaceous period is not fully understood. Such massive amounts of chalky sediments have never since been deposited over a comparable period of time. The high biological productivity of the Cretaceous oceans also led to ideal conditions (20) for oil accumulation. Oil is formed when organic material trapped in sediments is slowly buried and subjected to increased temperatures and pressures, transforming it into petroleum. Sediments rich in organic material accumulated along the margins of the Tethys Seaway, the tropical east-west ocean that formed when Earth's single landmass (known as Pangaea) split apart during the Mesozoic era. Many of today's important oil (25) fields are found in those sediments--in Russia, the Middle East, the Gulf of Mexico, and in the states of Texas and Louisiana in the United States.
【題組】31. What does the passage mainly discuss?
(A) How sediments were built up in oceans during the Cretaceous period
(B) How petroleum was formed in the Mesozoic era
(C) The impact of changes in oceanic animal and plant life in the Mesozoic era
(D) The differences between plankton found in the present era and Cretaceous plankton
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答案:A
難度:非常困難
【題組】33. The passage mentions all of the following aspects of plankton EXCEPT
(A) the length of their lives
(B) the level of the ocean at which they are found
(C) their movement
(D) their size
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答案:A
難度:非常困難
【題組】35. According to the passage, the most dramatic change to the oceans caused by plankton during the Cretaceous period concerned
(A) the depth of the water
(B) the makeup of the sediment on the ocean floor
(C) the decrease in petroleum-producing sediment
(D) a decline in the quantity of calcareous ooze on the seafloor
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答案:B
難度:適中
【題組】36. The "white chalk cliffs of Dover" are mentioned in line 14 of the passage to
(A) show where the plankton sediment first began to build up
(B) provide an example of a plankton buildup that scientists cannot explain
(C) provide an example of the buildup of plankton sediment
(D) indicate the largest single plankton buildup on Earth
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答案:C
難度:適中
Question 20-30 It is estimated that over 99 percent of all species that ever existed have become extinct. What causes extinction? When a species is no longer adapted to a changed environment, it may perish. The exact causes of a speciesdeath vary from situation Line to situation. Rapid ecological change may render an environment hostile to a species. For example, temperatures may change and a species may not be able to adapt. Food Resources may be affected by environmental changes, which will then cause problems For a species requiring these resources. Other species may become better adapted to an Environment, resulting in competition and, ultimately, in the death of a species. The fossil record reveals that extinction has occurred throughout the history of Earth. (10) Recent analyses have also revealed that on some occasions many species became extinct at the same time—a mass extinction. One of the best-known examples of mass extinction occurred 65 million years ago with the demise of dinosaurs and many other forms of life. Perhaps the largest mass extinction was the one that occurred 225 million years ago, When approximately 95 percent of all species died, Mass extinctions can be caused by (15) a relatively rapid change in the environment and can be worsened by the close interrelationship of many species. If, for example, something were to happen to destroy much of the plankton in the oceans, then the oxygen content of Earth would drop, affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction. One interesting, and controversial, finding is that extinctions during the past 250 Million years have tended to be more intense every 26 million years. This periodic extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but this theory is purely speculative. Some researchers have also speculated tat extinction may often be random. That is, certain species may be eliminated and others may survive (25) for no particular reason. A speciessurvival may have nothing to do with its ability or inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially random events.
【題組】25. Why is “ planktonmentioned in line 17?
(A) To demonstrate the interdependence of different species
(B) To emphasize the importance of food resources in preventing mass extinction.
(C) To illustrate a comparison between organisms that live on the land and those that live in the ocean
(D) To point out that certain species could never become extinct.
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答案:A
難度:適中