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当前学科：天然药物化学
题目：
选择题
从中药桂枝的乙酸乙酯提取物中分离得到一无色细针状结晶，mp.67—68℃，有特异香味，Emerson反应呈红色，则此结晶可能为（ ）
A. 6-羟基香豆素
B. 7，8-二羟基香豆素
C. 6，7-二羟基香豆素
D. 芦丁
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Questions 26 to 30 are based on the following passage. Exercise is one of the few factors with a positive role in long-term maintenance of body weight. Unfortunately, that message has not gotten through to the average American, who would rather try switching to “light” beer and low-calorie bread than increase physical exertion. The Centers for Disease Control, for example, found that fewer than one-fourth of overweight adults who were trying to shed pounds said they were combining exercise with their diet. In rejecting exercise, some people may be discouraged too much by caloric-expenditure charts: for example, one would have to briskly walk three miles just to work off the 275 calories in one delicious Danish pastry (小甜饼). Even exercise professionals concede half a point here. “Exercise by itself is a very tough way to lose weight,” says York Onnen, program director of the President’s Council on Physical Fitness and Sports. Still, exercise’s supporting role in weight reduction is vital. A study at the Boston University Medical Center of overweight police officers and other public employees confirmed that those who dieted without exercise regained almost all their old weight, while those who worked exercise into their daily routine maintained their new weight. If you have been sedentary (极少活动的) and decide to start walking one mile a day, the added exercise could burn an extra 100 calories daily. In a year’s time, assuming no increase in food intake, you could lose ten pounds. By increasing the distance of your walks gradually and making other dietary adjustments, you may lose even more weight. Some people dislike exercise because （）
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