【单选题】

Perhaps 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 broadone, designating all of the small plants and animals that float about or weakly propel(5) themselves through the se

A、In the late stages of the Mesozoic er
A、during theCretaceousperiod, 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 theEarth’’s longhistory, very large quantities of silica skeletons, which would eventually harden into rock,began to pile up in parts of the deep se
A、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 ofDover, in the southeast ofEngland, are just one(15)example of the huge quantities of such material that amassed during theCretaceousperiod; there are many more. Just why the calcareous plankton were so prolific duringthe latter part of theCretaceous 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 theCretaceous 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 whenEarth’’s single landmass(known as Pangaea) split apart during the Mesozoic er
A、Many of today’’s important oil(25)fields are found in those sediments--in Russia, the MiddleEast, the Gulf of Mexico, andin the states of Texas and Louisiana in the United States.The word "ideal" in line 19 is closest in meaning to
A、common
B．clear
C．perfect
D．immediate