单选题 In the second half of each year, many powerful storms are born in the tropical Atlantic and Caribbean Sea. Of these, only about half a dozen generate the strong, circling winds of 75 miles per hour or more that give them hurricane status, and several usually make their way to the coast. There they cause millions of dollars of damage, and bring death to large numbers of people.
The great storms that hit the coast start as innocent circling disturbances hundreds or thousands of miles out to sea. They travel aimlessly over water warmed by the trade winds. When conditions are just right, warm moist air flows in at the bottom of such a disturbance, moves upward through it and comes out at the top. In the process, the moisture in this warm air produces rain, and with it the heat is changed into energy in the form of strong winds. As the heat increases, the young hurricane begins to swirl in a counterclockwise (逆时针方向) motion.
The average life of a hurricane is only about nine days, but it contains almost more power than we can imagine. The energy in the heat released by a hurricane"s rainfall in a single day would satisfy the entire electrical needs of the United States for more than six months. Water, not wind, is the main source of death and destruction in a hurricane. A typical hurricane brings 6 to 12 inch downpours resulting in sudden floods. Worst of all is the powerful movement of the sea—mountains of water moving toward the low-pressure hurricane center. The water level rises as much as 15 feet above normal as it moves toward shore.

单选题 When is an ordinary tropical storm considered to be a hurricane?

单选题 What is the worst thing about the hurricanes?

单选题 The counterclockwise swirling of the hurricane is brought about by ______.

单选题 Apparently the phrase "innocent circling disturbances" in paragraph 2 means ______.

单选题 Which of the following best summarizes the passage?