12月英语四六级考试必背优秀范文(17)

1、姓名：_ 班级：_ 学号：_-密-封 -线- 12月英语四六级考试必背优秀范文(17)考试时间：120分钟 考试总分：100分题号一二三四五总分分数遵守考场纪律，维护知识尊严，杜绝违纪行为，确保考试结果公正。33 plankton浮游生物. / plktn; plktn/scattered through the seas of the world are billions of tons of small plants and animals called plankton. most of these plants and animals are too small for the hum

2、an 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 outweig

3、hs 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 seas plankton generates more than twice as much.despite its enormous food potential, little effect was made until recently to farm plankton

4、as we farm grasses on land. now marine scientists have at last begun to study this possibility, especially as the seas resources loom even more important as a means of feeding an expanding world population.no one yet has seriously suggested that “ plankton-burgers” may soon become popular around the

5、 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 provides the

6、major food for the great blue whale, the largest animal to ever 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.34 raising oystersin the oysters were raised in much the same

7、way as dirt farmers raised tomatoes- by transplanting them. first, farmers selected the oyster bed, cleared the bottom of old shells and other debris, then scattered clean shells about. next, they ”planted” fertilized oyster eggs, which within two or three weeks hatched into larvae. the larvae drift

8、ed until they attached themselves to the clean shells on the bottom. there they remained and in time grew into baby oysters called seed or spat. the spat grew larger by drawing in seawater from which they derived microscopic particles of food. before long, farmers gathered the baby oysters, transpla

9、nted them once more into another body of water to fatten them up.until recently the supply of wild oysters and those crudely farmed were more than enough to satisfy peoples needs. but today the delectable seafood is no longer available in abundance. the problem has become so serious that some oyster

10、 beds have vanished entirely.fortunately, as far back as the early 1900s marine biologists realized that if new measures were not taken, oysters would become extinct or at best a luxury food. so they set up well-equipped hatcheries and went to work. but they did not have the proper equipment or the

11、skill to handle the eggs. they did not know when, what, and how to feed the larvae. and they knew little about the predators that attack and eat baby oysters by the millions. they failed, but they doggedly kept at it. finally, in the 1940s a significant breakthrough was made.the marine biologists discovered that by raising the temperature of the water, they could induce oysters to spawn not only in the summer but also in the fall, winter, and spring. later they developed a technique for feeding the larvae and rearing them to spat. going still further, they succeeded in breeding new strains