听力原文：In the second half of each year, many powerful storms are born in the tropical At

听力原文： In the second half of each year, many powerful storms are born in the tropical Atlantic and Caribbean seas. Of these, only about a 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—even thousands—of miles out to sea. As they travel aimlessly over water warmed by the summer sun, they are carried westward 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 that is converted to energy in the form. of strong winds. As the heat increases, the young hurricane begins to swirl in a counter-clockwise motion.

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—the 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.

When is an ordinary tropical storm called a hurricane？

A．When it begins in the Atlantic and Caribbean seas.

B．When it hits the coastline.

C．When it is more than 75 miles wide.

D．When its winds reach 75 miles per hour.