职称英语 (12)
最新试题
She will be pleasedto meet you（）
02-21
She is sick（）
02-21
Jack was dismissed（）
02-21
We should not sacrifice environment to fostereconomic growth（）
02-21
Michael is now merelya good friend（）
02-21
A small number of firms have ceasedtrading（）
02-21
On the table was a vase filled with artificialflowers（）
02-21
I remember lots ofthings（）
02-21
I rarelywear a raincoat because I spend most of my time in a car（）
02-21
He is a physician（）
02-21
热门试题
Will you please call my husband as soon as possible?
She only needs a minute amount of money.
The governor gave a rather vague outline of his tax plan.
The book took ten years of thorough research.
共用题干
VisionHuman vision like that of other primates（灵长目）has evolved in an arboreal（丛林）environment.______(46)In the course of evolution,members of the primate line have acquired large eyes while the nose has shrunk.______ (47) Of mammals（哺乳动物）only humans and some primates enjoy color vision.______(48)Horses live in a single-color world.Light visible to human eyes,however,occupies only a very narrow band in the whole electromagnetic spectrum（光谱）.Ultraviolet rays（紫外线）are invisible to humans though ants and honeybees are sensitive to them.______(49)The world would look terribly different if human eyes were、 sensitive to infrared radiation（红外线）.Then instead of the darkness of night,we would be able to move easily in a strange shadowless world.______(50)But human eyes excel in other ways. They are in fact remarkably discerning in color gradation. The color sensitivity of normal human vision is rarely surpassed even by complicated technical devices.
There is a growing gap between the rich and the poor.
The only way to cure your toothache is to extract your bad tooth.
I wish to propose a toast to our friendship.
As the industry developing in these years,most of the rivers have been contaminated by sewage （污水）.
共用题干
Black HolesMost scientists agree that black holes exist but are nearly impossible to locate.A black hole in the universe is not a solid object, like a planet, but it is shaped like a sphere（球体）. Astronomers（天文学家）think that at the center of a black hole there is a single point in space with infinite（无限的）density（稠密）.This single point is called a singularity（奇点）.If the singularity theory is correct,it means that when a massive star collapses,all the material in it disappears into the singularity.The center of a black hole would not really be a hole at all,but an infinitely dense point.Anything that crosses the black hole is pulled in by its great gravity.Although black holes do exist,they are difficult to observe.These are the reasons.No light or anything else comes out of black holes.As a result,they are invisible to a telescope.In astronomical terms,black holes are truly tiny.For example,a black hole formed by the collapse of a giant star would have an event horizon（视界）only 18 miles across.The nearest black holes would be dozens of light years away from Earth.One light year is about 6 trillion（万亿）miles. Even the most powerful telescopes could not pick out an object so small at such a great distance.In 1994 the Hubble Space Telescope provided evidence that black holes exist. There are still answers to be found,however,so black holes remain one of the mysteries of the universe.
