题干

已知某种限制性内切酶在一线性DNA分子上有3个酶切位点,如图中箭头所指。如果在该线性DNA分子在3个酶切位点上都被该酶切断,则会产生a、b、c、d四种不同长度的DNA片段。现有多个上述线性DNA分子,若在每个DNA分子上至少有1个酶切位点被该酶切断,则从理论上讲,经该酶酶切后,这些线性DNA分子最多能产生长度不同的DNA片段种类数是(     )

A:3

B:4

C:9

D:12

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答案(点此获取答案解析)

C

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    Too much work and too much wine go together like biscuits and cheese, a new international studies finds. Employees who work more than 48 hours a week are 11 percent more likely to over-consume (过渡消费) alcohol than those who work standard time, Finnish researchers say. The study looked at more than 300,000 people in Australia, Europe and North America. No differences were seen between men and women, says the study, published in the British Medical Journal.

    Risky alcohol consumption is considered as more than 14 drinks a week for women and more than 21 drinks a week for men. About 20 percent of Australians drink at levels that put them at risk of lifetime harm from injury or disease. Drinking alcohol can affect the liver or cause brain damage, heart disease, high blood pressure and increase the risk of many cancers.

    Study author Marianna Virtanen said while alcohol might help ease the stress of working long periods of time, risky consumption could lead to difficulties in the workplace, such as poor performance. The European Union Working Time Directive ensures that workers in EU countries have the right to work no more than 48 hours a week, including overtime. “ But many people , for example well-educated managers and professionals, work much longer hours to achieve faster promotions (晋升), salary increases, and more control over work and employment,” said Prof. Virtanen from the Finnish Institute of Occupational Health.