Câu hỏi:

Read the following passage and mark the letter A, B,C or D on your answer sheet to indicate the correct answer to each of the following questions.         The people of Kiribati are afraid that one day their country will disappear from the face of the earth. Several times this year, this nation has been flooded by a sudden high tide. These tides, which swept across the island and destroyed houses, came when there was a neither wind nor rain. This never happened before.          What is causing these mysterious high tides? The answer may is global warming. When fuels like oil and coal are being burned, pollutants are released; these pollutants trap heat in the earth’s atmosphere. Warmer temperatures cause water to expand and also create more water by melting glaciers and polar ice caps.           If the trend continues, scientist say, many countries will suffer. Bangladesh, for example, might lose one-fifth of its land. The coral island nations of the Pacific, like Kiribati and the Marshall Islands, however would face an ever worse fate- they would be swallowed by the sea. 

The word “happened” in the first paragraph is best replaced by______.