A、Global warming. B、Polluted seawaters. C、Commercial hunting. D、Decreasing birthrates. C细节辨认题。短文中提到，鲸鱼数量濒危是因为近年来的商业捕杀活动，过度捕猎可能导致

An endangered species of whale is experiencing a small-baby boom off the coast of America. The North Atlantic right whale is one of the rarest species of whale on the planet, numbering only about 411. But the Center for Coastal Studies said Friday that its aerial survey team spotted a mum with two babies in Cape Cod Bay a day earlier. That brings the number seen in nearby waters alone this year to three. That’s big news because the whale population has been falling, and no baby whales were seen last year. In all, seven baby whales have been spotted so far this year. The whale population has become endangered due to commercial whaling activities in recent years. This is because they are sometimes hunted for their meat or their skin. Over-hunting could lead to the disappearance of the whale population, possibly causing major problems to the global food chain. The whales give birth off the Southeast Coast of America in the winter and travel to feeding grounds off the Northeast Coast in the early spring. The Northeast Coast is a critically important source of food. The animals often feed close to shore. This provides watchers on land with “unbeatable views of one of the rarest of marine mammals.” It’s illegal to get within 1,500 feet of the animals without a federal research permit, so whale watchers are discouraged from attempting to get close to the whales.
Questions 19 to 21 are based on the passage you have just heard.
19. What do we learn from the passage about the North Atlantic right whale?
20. What has caused the decline of the whale population in recent years?
21. Why do the whales travel to the Northeast Coast of America in the early spring?