单选题

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 non- melanoma (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—the strongest effect appears to be in 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 benefits, including pain relief, increased alertness, and increased physical endurance, have also been attributed to caffeine. As a component of numerous over the counter diet pills and pain relievers, caffeine increases their effectiveness and helps the body absorb them more quickly. By constricting (收缩) blood vessels in the brain, it can alleviate headaches—even migraines (偏头痛)—and can help counter the drowsiness (眩晕) caused by antihistamines (抗组胺药).
Caffeine does not alter the need for sleep, but it does offer a temporary solution to fatigue for people who need to stay alert. Research has shown that sleep deprived individuals who consumed caffeine had improved memory and reasoning abilities, at least in the short term. Studies of runners and cyclists have shown that caffeine can improve their stamina—hence its addition to energy boosting sports drinks.
People who consume a lot of caffeine regularly may develop temporary withdrawal symptoms, headache being the most common, if they quit or cut back on it abruptly. Fortunately, these symptoms last only a day or two in most cases. Individuals who are more sensitive to the stimulatory side effects of caffeine may want to avoid it, but most doctors agree that the equivalent of three cups 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.

单选题 Drinking coffee or tea may help ______.

【正确答案】 B

【答案解析】[解析] 根据文章第一段第三句“Habitual coffee and tea drinkers had long been observed to have a lower incidence of non-melanoma skin cancers”可知，常喝咖啡和茶的人得皮肤癌的概率低，所以B项正确。A项文中并未提及。根据文章第一段第六句“Drinking caffeinated coffee has also been associated with a decreased incidence of endometrial cancer”可知，常喝咖啡与子宫内膜癌发病率的降低有相关性，所以C项错误。根据文章第一段最后一句“caffeinated coffee has also been associated with lowered risk of cirrhosis and liver cancer”可知，咖啡因也与降低患肝硬化和肝癌的风险有关，所以D项错误。故本题选B。

单选题 Caffeine is used to ______.

【正确答案】 B

【答案解析】[解析] 根据文章第二段倒数第一、二句“As a component...caffeine increases their effectiveness and helps the body absorb them more quickly. By constricting blood vessels in the brain, it can alleviate headaches...”可知，咖啡因有助于止痛药的吸收，且通过脑内血管收缩能够缓解头痛，所以B项正确。根据文章第二段第一句“many of caffeine"s undesirable effects, such as elevated...blood pressure”可知，咖啡因使血压升高，而非降低，所以A项错误。根据第一段第三句“Habitual coffee and tea drinkers...have a lower incidence of non-melanoma skin cancers”和最后一句“caffeinated coffee has also been associated with lowered risk of...liver cancer”可知，咖啡因与降低患皮肤癌和肝癌的风险有关，而文章并未提及可用咖啡因来治疗肝癌与皮肤癌，所以C项和D项错误。故本题选B。

单选题 Some athletes use caffeine to ______.

【正确答案】 B

【答案解析】[解析] 题目所给选项中，B项与文章第二段第一句“...some short-term benefits, including pain relief, increased alertness, and increased physical endurance”所叙相符，并且根据第三段最后一句“hence its addition to energy-boosting sports drinks”可知，将咖啡因加入运动员补充能量的饮料中，能增强耐力，所以B项正确。而A项与D项文章并未提及。C项有一定的迷惑性。由文章第三段第一、二句“Caffeine...does offer a temporary solution to fatigue for people who need to stay alert. Research has shown...improved memory and reasoning abilities”可知，咖啡因能够使疲劳的人保持清醒，且睡眠不好的人喝咖啡后其记忆力和理解力会得到改善，而这些所指的都不是运动员的情况，C项张冠李戴，故错误。所以本题选B。