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  3. Caffeine Caffeine is probably the most widely used drug in the world. Human...

Caffeine Caffeine is probably the most widely used drug in the world. Human...

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问题 Caffeine
Caffeine is probably the most widely used drug in the world. Humans have been consuming caffeine for hundreds of years, primarily in the form of coffee, tea, and cocoa. Habitual coffee and tea drinkers had long been observed to have a lower incidence of nonmelanoma (黑色素瘤）skin cancers, although no one knew why. A recent study found that caffeine affects skin cells damaged by ultraviolet radiation, a main cause of skin cancer. Caffeine interferes with a protein that cancerous cells need to survive，leaving the damaged cells to die before they become cancerous. Drinking caffeinated coffee has also been associated with a decreased incidence of endometrial (子宫内膜的)cancer — that is, cancer of the cells lining the uterus (子宫）. The strongest effect appears to be iin overweight women, who are at greatest risk for the disease. Researchers believe blood sugar, fat cells, and estrogen (雌性激素）may play a role. Although the mechanism remains unknown, people who drink more than two cups of coffee or tea a day reportedly have about half the risk of developing chronic liver disease as those who drink less than one cup of coffee daily； caffeinated coffee has also been associated with lowered risk of cirrhosis (肝硬化）and liver cancer.
While many of caffeine’s undesirable,effects such as elevated heart rate and blood pressure, are brief, some short-term been ncluding pain relief, increased alertness, and increased physical endurance, have JiPi^en attributed to caffeine. As a component of numerous over-the-counter diet piks and pain relievers, caffeine increases their effectiveness and helps the body absornfhem more quickly. By constricting (收缩)blood vessels in the brain，it can alleviate hffifaches — even migraines (偏头痛)—and can help counter the drowsiness (眩晕）causde by antihistamines (抗组胺药）.
Caffeine does not alter the neefjor sleep, bumit does offer a temporary solution to fatigue for people who need to alert. Research has shown that sleep-deprived individuals who consumed caffeine hMimproved memory and reasoning abilities, at least in the short term. Studies of runners am cyclists have^iown that caffeine can improve their stamina — hence its addition to ener -boosting drinks.
People who consume a lot of M8ine regularly may develop temporary withdrawal symptoms, headache being the mom If they quit or cut back on it abruptly. Fortunately, these symptoms last only in most cases. Individuals who are more sensitive to the stimulatory side may want to avoid it, but most doctors agree that the equivalent of threr^ups of coffee a day does not harm healthy people. There is no medical basis to give up daily caffeine and many reasons to include a moderate amount in one’s diet.
Some athletes use caffeine to
选项 A increase their endurance.
B improve their speed.
C maintain their alertness.
D relax their muscles.
答案A
解析第三段最后一句说，“对跑步和自行车运动员的研究表明，咖啡因可以提高他们的耐力”，而endurance的意思正是短文中stamina的意思，可见A项是正确答案，其他选项短文中均未提到。
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