In one of the world's most forbidding environments,Polar bears spend their summers roaming（漫步）the Arctic on large pieces of floating ice.They drift（漂移）for hundreds of miles,finding mates,hunting for seals,and fatting themselves up for the winter.Without the floating ice,the world's largest bears could not survive.Yet at this moment,the polar bears' Arctic habitat （栖息地） is melting away under it because of global warming.
Over the past three decades,more than a million square miles of floating ice has disappeared.Scientists predict that,if the current rate of global warming continues,most,if not all,of the bears' floating ice will be gone by 2100.As a result,the world's polar bears could face global dying out by the end of this century.
Already,the ice on the southern edge of the polar bears' habitat is melting about three weeks earlier than in the past.The loss of those key weeks leaves the bears less time to hunt,eat and store up fat.Once the ice melts, the bears must fast for up to eight months on land until the floating ice returns.As a result of early melting,there has been a 14 percent decline in the Hudson Bay polar bear population over the past ten years—a fall clearly caused by global warming.
Besides,a growing number of polar bears may be drowning（溺水）as they are forced to swim more often,and for longer distances,looking for ice sheets（冰原）.According to a report by the US Minerals Management Service,researchers observed four dead polar bears floating 60 miles off Alaska in September of 2006 and said it was likely that many other bears swimming far off shore also drowned.Scientists have concluded that the worldwide population of about 20，000 polar bears is likely to become smaller by more than 30 percent in the next 35 to 50 years alone.



What do polar bears usually do in summer？ ______

A. Look for relatives.
B. Hunt for food.
C. Store crops up.
D. Make friends with seals.

What disaster will happen to polar bears by the end of this century according to the text？ ______

A. Global drowning.
B. Global dying out.
C. Global hunger.
D. Global roaming.

What can be inferred from the underlined part in Paragraph 3？ ______

A. The bears will have to stay hungry longer.
B. The bears will have to roam faster.
C. The bears will have to stay quieter on land.
D. The bears will have to face a faster fall of its population.

Why do polar bears have to swim more often and farther？ ______

A. To search for food.
B. To build up bodies to survive.
C. To find places to land on.
D. To escape being hunted.