【正确答案】 B

【答案解析】[听力原文]
Canadian and American scientists studied the effects of people's fishing for sharks in the Northwest Atlantic over the past thirty-five years. Results of their studies were published in Science magazine.
The scientists say one effect of shark fishing has been an estimated ninety-nine percent decrease in some shark populations. They say the loss of larger sharks has caused a population explosion among fish like skates and rays. Such fish and smaller sharks have increased in number along the east coast of the United States.
Sharks usually eat skates and rays. The scientists say these fish feed on shellfish, which are disappearing from the ocean. They say other sea animals are also being threatened by the area's changing environment.
Demand for shark fins has been rising in Asia. Shark fins are used for medical purposes and also for food.
For one of the studies, scientists from Canada examined information from private fishing companies and other research projects. They noted a sharp decrease in eleven kinds of great sharks since the nineteen eighties.
One of the scientists was Julia Baum of Dalhousie University. She says the World Conservation Union earlier this year listed great hammerhead and scalloped hammerhead sharks as being in danger of disappearing. The group also reported the dusky and sandbar sharks as being threatened.