問題詳情

Questions 31-39Perhaps one of the most dramatic and important changes that took place in theMesozoic era occurred late in that era, among the small organisms that populate theuppermost, sunlit portion of the oceans--the plankton. The term "plankton" is a broadLine 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 Cretaceousperiod, there was a great expansion of plankton that precipitated skeletons or shellscomposed of two types of mineral: silica and calcium carbonate. This developmentradically 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 longhistory, 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 ofthe tiny remains of the calcium carbonate-secreting plankton also accumulated as neverbefore. 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 Cretaceousperiod; there are many more. Just why the calcareous plankton were so prolific duringthe latter part of the Cretaceous period is not fully understood. Such massive amountsof 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 slowlyburied and subjected to increased temperatures and pressures, transforming it intopetroleum. Sediments rich in organic material accumulated along the margins of theTethys 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, andin the states of Texas and Louisiana in the United States.
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

參考答案

答案：A
難度：非常困難0
統計：A(0),B(0),C(1),D(0),E(0)

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