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Một điện tích điểm q=1μC di chuyển từ điểm A đến điểm B trong điện trường, công của lực điện trong quá trình dịch chuyển điện tích q là A=0,2mJ. Hiệu điện thế giữa hai điểm A và B là – Học trắc nghiệm
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Read the following passage and mark the letters A, B, C or D on your answer sheet to indicate the correct answer to each of the questionsScientists do not yet thoroughly understand just how the body of an individual becomes sensitive to a substance that is harmless or even wholesome for the average person. Milk, wheat, and egg, for example, rank among the most healthy and widely used foods. Yet these foods can cause persons sensitive to them to suffer greatly. At first, the body of the individual is not harmed by coming into contact with the substance. After a varying interval of time, usually longer than a few weeks, the body becomes sensitive to it, and an allergy has begun to develop. Sometimes it’s hard to figure out if you have a food allergy, since it can show up so many different ways. Your symptoms could be caused by many other problems. You may have rashes, hives, joint pains mimicking arthritis, headaches, irritation, or depression. The most common food allergies are to milk, eggs, seafood, wheat, nuts, seeds, chocolate, oranges, and tomatoes. Many of these allergies will not develop if these foods are not fed to an infant until her or his intestines mature at around seven months. Breast milk also to be protective. Migraines can be set off by foods containing tyramine, phenathylamine, monosodium glutamate, or sodium nitrate. Common foods which contain these are chocolate, aged cheeses, sour cream, red wine, pickled herring, chicken livers, avocados, ripe bananas, cured meats, many Oriental and prepared foods (read the labels!). Some people have been successful in treating their migraines with supplements of B-vitamins, especially B6 and niacin. Children who are hyperactive may benefit from reducing food additives, especially colorings, and foods high in salicylates from their diets. According to the passage, the difficulty in diagnosing allergies to foods is due to ______________.