单选题The fireman had a ______ escape when a staircase collapsed beneath this feet.

单选题{{B}}Passage Four{{/B}} We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals too. In one experiment, for example, behavioral immunologist (免疫学家) Mark Laudenslager, at the University of Denver, gave mild electric shocks to 24 rats. Half the animals could switch off the current by turning a wheel in their enclosure, while the other half could not. The rats in the two groups were paired so that each time one rat turned the wheel it protected both itself and its helpless Partner from the shock. Laudenslager found that the immune response was depressed below normal in the helpless rats but not in those that could turn off the electricity. What he has demonstrated, he believes, is that lack of control over an event, not the experience itself, is what wakens the immune system. Other researchers agree. Jay Weiss, a psychologist at Duke University School of Medicine, has shown that animals who are allowed to control unpleasant stimuli don't develop sleep disturbances or changes in brain chemistry typical of stressed rats. But if the animals are conditioned to confront with situations they have no control over, they later behave passively even when faced with experiences they can control. Such findings reinforce psychologists' suspicions that the experience or perception of helplessness, is one of the most harmful factors in depression. One of the most startling examples of how the mind can alter the immune response was discovered by chance. In 1975 psychologist Robert Ader at the University of Rochester School of Medicine conditioned (便形成条件反射) mice to avoid saccharin (糖精) by simultaneously feeding them the sweetener and injecting them with a drug that while suppressing their immune systems caused stomach upsets. Associating the saccharin with the stomach pains, the mice quickly learned to avoid the sweetener. In order to extinguish this dislike for the sweetener, Ader reexposed the animals to saccharin, this time without the drug, and was astonished to find that those mice that had received the highest amounts of sweetener during their earlier conditioning died. He could only speculate that he had so successfully conditioned the rats that saccharin alone now served to weaken their immune systems enough to kill them.

单选题You have to finish the work within the ______ time.

单选题A compound break is more serious than a simple one because there is more opportunity for loss of blood and infection.

单选题(Breaking) up water (into) hydrogen and oxygen is a good example of (as) is known as (a chemical change).

单选题(That) the woman (was saying) was so important that I asked everyone (to stop) talking and (listen).

单选题Woman: It's your own fault. You shouldn't let anyone in unless you are expecting him.Man: It's all very well to say that, but someone comes to the door saying "electricity" or "gas" while presenting you a card, and you automatically think he is OK.Question: What does the woman mean?

单选题The National Institute of Mental Health is conducting a {{U}}far-reaching{{/U}} research to determine the psychological effects of using drug.

单选题Such questions should be approached honestly and in full awareness that______loan agreements will cost money due to cancellation or other charges.

单选题Usually you will be (more likely) to find insects (if) you examine finer twigs (rather than) the (coarse) parts of trees.

单选题Many countries have conservation programs to prevent certain ______ of fish from becoming extinct.

单选题Some criminal slang is so highly developed that pickpockets, for example, can ______ a conversation in front of a victim without the person's realizing that they are discussing.

单选题(The farmer) knows something that the whole (civilized) mankind seems (to have forgotten), namely, (when) the resources of life on our planet are not inexhaustible.

单选题Man: You know what? You should invest the money yourself. Woman: That had crossed my mind. Question: What does the woman mean?

单选题What does the word "concurrent" (Par

单选题Despite years of searching, scientists have detected no signs of life {{U}}beyond {{/U}} our own solar system.

单选题The courses aim to give graduates an up-to-date grasp of their subject and ______ laboratory skirls.

单选题Jennifer Senior suggests in her article that raising a child can bring ______.

单选题Wilmut maintains that cloning animals has tremendous potential for helping people. Cloned Sheep, he says, could be used as living drug factories. Scientists could "engineer" sheep that produce drugs in their milk. And by altering the proteins on the surfaces of animal organs to make them more like human organs, scientists believe they may be able to create a plentiful source of organ donors for people. Why not clone humans as organ donors? Theoretically, Wilmut says, there is no reason his techniques couldn't someday be used to clone people. Think about the possibilities: a whole basketball team of Michael Jordans, a scientific panel of Albert Einsteins, a movie starring and co-starring Brad Pitts. On a more serious note, some experts argue that couples who have difficulty having a baby could make copies of themselves. And parents whose child has a fatal disease like cancer might be able to clone the child, creating a twin who could be a bone-marrow (骨髓) donor. But even Ian Wilmut draws the line at cloning humans. "All of us would find that offensive," he says. Several countries, including Britain, Denmark, Germany and Australia, have made all scientific work on cloning humans illegal. The U.S. has no such law, but President Clinton has set up a panel of scientists and philosophers to study the issue. In the meantime, Clinton has imposed a ban on using federal money to clone humans. Humans are more than the sum of their genes, argues a philosopher at one research institute. Though they look exactly the same, clones are not necessarily exact copies. The younger twin might grow up with different influences—say, unusual friends or special teachers. A cloned Albert Einstein might fail his physics class. A cloned pop star might sing terribly. Say you were cloned. Would your twin live a shorter life because he or she started out with DNA that was already 10, 20 or 30 years old? Scientists aren't sure. And how could you prevent someone from taking a sample of your hair and making a clone of you? Again, no solutions. What do you think? Should scientists be allowed to clone animals? How about humans?