A、Polluted water. B、Decreasing fish. C、Climate change. D、Over-hunting. C选项中的Polluted water，Climate change，Over-hunting等表明，本题可能考查

In Spring 2008, the polar bear was placed on the endangered species list. According to the Endangered Species Act, an endangered species is an animal that is likely to face extinction in its natural habitat. Polar bears have been categorized as a "threatened" species. The ESA defines a threatened species as one that is likely to become "endangered" in the foreseeable future. The polar bear is the first animal that has been classified as endangered due primarily to global warming. Global warming is caused by carbon dioxide and other greenhouse gases that become trapped in the atmosphere. Heating homes, driving cars, and burning garbage all require fossil fuels that lead to global warming. The polar bear’s habitat is more vulnerable to global warming than many other species. Polar bears live mainly on the sea ice in the Arctic. This is where they hunt for fish and build up fat reserves. When the ice melts many polar bears move to land and live off their stored fat. In the Arctic, global warming is causing the ice to melt slightly earlier and form slightly later. This results in a shorter feeding season for the polar bear. Some risk their lives to find ice. If they have to swim too far they will drown from exhaustion and hunger. The World Wildlife Fund estimates that nearly 25% of the Arctic sea ice has disappeared in the past 30 years. Since the Endangered Species Act was introduced in the U.S. in 1973, many species have been taken off the list due to increased populations.
Questions 29 to 31 are based on the passage you have just heard.
29. What is an endangered species, according to the passage?
30. What poses the greatest threat to polar bears today?
31. What percentage of the Arctic sea ice has disappeared in the past 30 years?