【正确答案】 B

【答案解析】[听力原文] Floods, storms and other natural events kill thousands of people every year. So does extreme heat. In fact, experts say heat may be nature's deadliest killer. Several hot days that follow each other are considered a heat wave. Experts say heat waves often become dangerous when the nighttime temperature does not drop much from the highest daytime temperature. This puts great pressure on the human body. Over the past month, the northeastern United States has been very hot. Amtrak, the passenger railroad service, announced on July 18th that it was reducing the speed at which its trains travel in the Northeast. The reason is extreme heat. Amtrak said it made the decision after the temperature inside some train tracks rose above 48.9 degrees Celsius. On the other side of the world, some people died in Shanghai, China during the city's hottest July in 140 years. The temperature in several Chinese provinces hit over 39 degrees Celsius. One television station showed images of a piece of meat cooking in the street to demonstrate the extreme heat. The China Meteorological Administration warned people to limit outdoor activities. In Australia, record-setting heat fueled bush fires across Tasmania earlier this year. The highest temperature, 49.6, was recorded in Moomba, South Australia. Doctors say people can do many things to protect themselves from the dangers of extreme heat. Stay out of the sun, if possible. Drink lots of cool water. Wear light-colored clothing and hats made of natural materials. Make sure the clothing is loose, permitting freedom of movement. According to the experts, when do heat waves often become dangerous?