题干

从欧姆定律可以导出公式R=U/I,下列说法正确的是  ()

A:当电压U增大为原来的2倍时,电阻R也增大为原来的2倍

B:当电流I增大为原来的2倍时,电阻R减小为原来的二分之一

C:通过导体的电流若为零,电阻也为零

D:即使导体两端的电压为零,电阻也不为零

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D

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    A drug that is used to treat serious bleeding could save thousands of lives of mothers giving birth. A world test of the drug found it reduced (降低) the risk of bleeding deaths during childbirth by nearly one third. The study included 20,000 women in 21 countries, mainly in Africa and Asia. The test was carried out by London School of Hygiene and Tropical Medicine. The drug is called tranexamic acid (凝血酸). It is low-cost and researchers said it does not cause serious side effects (副作用) for mothers or babies.

    The drug is used to treat mothers for serious bleeding during childbirth. Postpartum hemorrhage (产后出血) is the main cause of mothers' death around the world, according to the World Health Organization.

    Haleema Shakur is the project director for the test. She said though medical condition is improved in many countries, serious bleeding after childbirth remains a big problem in some parts of the world. “It's one of the biggest killers of mothers. In Africa and Asia, about 10% of women will end up with serious bleeding.” The drug works after a mother gives birth.

    Shakur said the best results were reported when the drug was given to women as soon as possible after childbirth. “The earlier you give it — within the first three hours after giving birth — the better the result is.”

    The next step will be to provide training for doctors and nurses on how to use it and then to find where the drug is needed. During the test, women were given the drug in a hospital. But researchers are looking for easier ways to use the drug so it can be more widely used in small clinics and the countryside.