单选题Why it is difficult to say exactly how many miles of improved waterways are in operation?

单选题WhatwasTim'sfirstjob?

单选题 As people age, their cells become less efficient and less able replace damaged components. At the same time their tissues stiffen, For example the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten. Few investigators would attribute such diverse effects to a single cause. Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age-related impairment of both cells and tissues. That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes. When enzymes attach glucose to proteins( enzymatic glycosylation), they do so at a specific site on a specific protein molecule for a specific purpose. In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule. This nonenzymatic glycosylation of certain proteins has been understood by food chemists for decades, although few biologists recognized until recently that the same steps could take place in the body. Nonenzymatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (HN2) of a protein are attracted to each other. The molecules combine, forming what is called a Schiff base within the protein. This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product. If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures. These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products ( AGE’s). Most AGE’s are yellowish brown and fluorescent and have specific spectrographic properties. More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs. Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues. In an attempt to link this process with the development of cataracts ( the browning and clouding of the lens of the eye as people age), researchers studied the effect of glucose on solutions of purified crystallin, the major protein in the lens of the eye. Glucose-free solutions remained clear but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked. The clusters diffracted light, making the solution opaque. The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGE’s. These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation.

单选题The author mentions contraception to demonstrate that______.

单选题The colonists who first came and settled in Canada are from A. the Ｕ. Ｋ. B. Spain. C. the Ｕ. Ｓ. Ａ. D. France.

单选题Which of the following is likely to be a topic of the Oprah Winfrey show?

单选题______, written by David Herbert Lawrence, is regarded as a typical example and lively manifestation of Oedipus Complex in fiction.A. The White Peacock B. Lady Chatterley's LoverC. The Rainbow D. Sons and Lovers

单选题______ is the scientific study of language, studying not just one language of any one society, but the language of all human beings.[A] Linguistics[B] Pragmatics[C] Semantics[D] Syntax

单选题The" paradoxes" in the passage mean______

单选题______was a man who lived out the American Dream.

单选题The Deserted Village was written by______, the outstanding representative of the sentimentalist school.

单选题There are ______ morphemes in the word "uncomfortably". A. three B. four C. five D. six

单选题Beforesetuphispizzadeliveryservice,he

单选题Pride and Prejudice was written by _______.A. Jane Austen B. Emily BronteC. Jane Eyre D. Charlotte Bronte

单选题The word "exceeded" in paragraph 1 most probably means_____.

单选题{{I}} Questions 1 to 5 are based on an interview. At the end of the interview you will be given 10 seconds to answer each of the following five questions. Now listen to the interview.{{/I}}

单选题What people brought the new religion Christianity to Britain first?

单选题Questions 9 and 10 are based on the following news. At the end of the news item, you will be given 20 seconds to answer the questions.

单选题______ does not belong to The Great Lakes.

单选题{{B}}TEXT C{{/B}} Legends about King Arthur have existed since the 6th century. Stories of the man and his doings have grown far beyond anything that could be regarded as factual history. Here are some of the highlights. Arthur was born as a result of the wizardry of Merlin, who arranged all adulterous liaison between Arthur's father, King Uther Pendragon, and his lover, a married duchess. Merlin agreed to do this only if the lovers allowed him to bring up the child born of the affair. When Uther Pendragon died some years later, there was confusion in the kingdom about who should inherit the throne. Merlin arranged a pageant where many knights came to try their luck at pulling a sword out of a stone. Whoever successfully extracted the blade was the rightful king. After many a brave knight had tried and failed, Merlin presented the young Arthur who, to everyone's surprise, easily pulled out the sword. As king, Arthur established the knightly fellowship of the Round Table at his castle of Camelot, so appear all the other chivalrous knights associated with the king. The knights of the Round Table spent much of their time on the quest for the Holy Grail. The Grail is the cup used by Jesus at the Last Supper, which was allegedly brought to Britain, then somehow lost. It is notoriously hard to get hold of, as finding it requires an almost superhuman degree of moral purity. At last it was the true gentleman Sir Galahad who eventually found it and set off to return it to its rightful place in the Holy Land. Arthur's death is a matter of some debate. According to legend, one of Arthur's less intelligent moves was his decision to marry the Lady Guinevere, who fell in love with Sir Lancelot, and their adultery led to war among the knights of the Round Table, culminating in the Battle of Camlan and Arthur's mortal woound. After the Battle of Catalan the wounded king was taken to the mysterious isle of Avalon ruled by his sister Morgan Le Faye. She, being skilled in the arts of witchcraft and healing, was apparently meant to cure him. But evidently Arthur thought he had little chance, because he gave his sword, Excalibur, to Sir Bedivere to return to the Lady of the Lake, an enigmatic character from whom Arthur had originally received the blade. Bedivere hurled the sword over the water, where a spooky hand appeared from the lake to catch it, waved it around for a while and then carried it down to the murky depths where, who knows, perhaps it still lies. As for Arthur, we can only conclude that his sister wasn't such a good doctor.