2014年4月27日托福考前冲刺题(一)_第5页

　　Scattered through the seas of the world are billions of tons of small plants and animal scalled plankton. Most of these plants and animals are too small for the human eye to see. They drift about lazily with the currents, providing a basic food for many larger animals.

　　Plankton has been described as the equivalent of the grasses that grow on the dry land continents, and the comparison is an appropriate one. In potential food value, however, plankton far outweighs that of the land grasses. One scientist has estimated that while grasses of the world produce about 49 billion tons of valuable carbohydrates each year, the sea's plankton generates more than twice as much.

　　Despite its enormous food potential, little effort was made until recently to farm plankton as we farm grasses on land. Now, marine scientists have at last begun to study this possibility, especially as the sea's resources loom even more important as a means of feeding and expanding world population.

　　No one yet has seriously suggested that "planktonburgers" may soon become popular around the world. As a possible farmed supplementary food source, however, plankton is gaining considerable interest among marine scientists.

　　One type of plankton that seems to have great harvest possibilities is a tiny shrimp-like creature called krill. Growing to two or three inches long, krill provide the major food for the giant blue whale, the largest animal ever to inhabit the Earth. Realizing that this whale may grow to 100 feet and weigh 150 tons at maturity, it is not surprising that each one devours more than one ton of krill daily.

　　Krill swim about just below the surface in huge schools sometimes miles wide, mainly in the cold Antarctic. Because of their pink color, they often appear as a solid reddish mass when viewed from a ship or from the air. Krill are very high in food value.

　　If the krill can feed such huge creatures as whales, many scientists reason, they must certainly be contenders as a new food source for humans.

　　30.Which of the following statements best describes the organization of the passage?

　　(A) The author presents the advantages and disadvantages of plankton as a food source.

　　(B) The author quotes public opinion to support the argument for farming plankton.

　　(C) The author classifies the different food sources according to amount of carbohydrate.

　　(D) The author makes a general statement about plankton as a food source and then moves to a specific example.

　　PASSAGE 8

　　By long-standing convention, all meteorites are assigned to three broad divisions on the basis of two kinds of material that they contain: metallic nickel-iron (metal) and silicates, which are compounds of other chemical elements with silicon and oxygen. As their name suggests, the iron meteorites consist almost entirely of metal. At the opposite extreme, the stony meteorites consist chiefly of silicates and contain little or no metal. A third category, stony-irons, includes those meteorites that contain similar amounts of metal and silicates. Since meteoritic metal weighs more than twice as much as the same volume of meteoritic silicates, these three kinds of meteorites can usually be distinguished by density, without more elaborate tests.

　　The stony meteorites can also be subdivided into two categories by using nothing more complicated than a magnifying glass. The great majority of such meteorites are chondrites, which take their name from tiny, rounded objects-chondrules-that occur in most of them and are among their most puzzling features. The rest of the stony meteorites lack chondritic texture and are therefore called achondrites. Achondrites vary widely in texture, composition, and history.

　　Irons, stony-irons, chondrites, and achondrites are by no means equally abundant among observed meteorites: chondrites are much more common than all other kinds of meteorite sput together. The irons, which are usually prominent in museum displays, are really quite uncommon. Curators like to highlight iron meteorites because many of them are large and their internal structure is spectacular in polished, etched slices. A stony meteorite has a beauty of its own, but it only appears under the microscope: to the unaided eye, stony meteorites appear to be--indeed they are--rather homely black or gray rocks.

　　To go further with meteorite classification, it is necessary to be more specific about the minerals that make up a meteorite: which silicates are present, and what kind of metal? To answer these questions, one needs to see more detail than is visible to the unaided human eye.

　　31. Which of the following diagrams most clearly shows the organization of the passage?