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 questions from 30 to 34.

        A tropical cyclone is a violent low pressure storm that usually occurs over warm oceans of over 80°F or 27°C. It winds counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as it is described for the term, cyclone itself. This powerful storm is fueled by the heat energy that is released when water vapor condenses at high altitudes, the heat ultimately derived from the Sun.

        The center of a tropical cyclone, called the eye, is relatively calm and warm. This eye, which is roughly 20 to 30 miles wide, is clear, mainly because of subsiding air within it. The ring of clouds around the eye is the eyewall, where clouds reach highest and precipitation is heaviest. The strong wind, gusting up to 360 kilometers per hour, occurs when a tropical cyclone’s eyewall passes over land.

        There are various names for a tropical cyclone depending on its location and strength. In Asia, a tropical cyclone is named according to its strength. The strongest is a typhoon; its winds move at more than 117 kilometers per hour. In India, it is called a cyclone. Over the North Atlantic and in the South Pacific, they call it a hurricane.

        On average, there are about 100 tropical cyclones worldwide each year. A tropical cyclone peaks in late summer when the difference between temperature in the air and sea surface is the greatest. However, it has its own seasonal patterns. May is the least active month, while September is the most active.

        The destruction associated with a tropical cyclone results not only from the force of the wind, but also from the storm surge and the waves it generates. It is born and sustained over large bodies of warm water, and loses its strength over inland regions that are comparatively safe from receiving strong winds. Although the tract of a tropical cyclone is very erratic, the Weather Service can still issue timely warnings to the public if a tropical cyclone is approaching densely populated areas. If people ever experience a cyclone, they would know how strong it could be.

What is the main idea of the passage?