compete with Arctic seals for food supplies from fish. Lack of polar bears could eventually affect human food supply.Human beings shall save polar bears to ensure the food supply.

　　There is no doubt that we should save polar bears.However，there is still another question: can they be saved? This is a matter that scientists debate because of rapid degradation (恶化) of the polar bears' environment in recent years due to warming temperatures. It is estimated that if the bears cannot adapt to changing climates，they might die out before the end of the 21st century，with about 50% of the animals disappearing by 2050.So time is running out!

　　There are some things that may help slow down global warming and help delay extinction (灭绝) of polar bears. These include finding new energy sources that don't give off greenhouse gases and not making any more incursions into polar bear's territory.

　　[语篇解读]　本文为议论文。作者在文中阐述了保护北极熊的重要性。

1．Which is NOT the threat to the survival of polar bears according to the passage?

A．Habitat loss.　　　　　 B．Global warming.

C．Human's killing. D．Lack of food.

解析：细节理解题。根据第一段第二、三句"They are dying because they are losing a great deal of habitat due to human incursion(侵入)and global warming.They are dying because of starvation as well."可知，栖息地丧失、全球变暖及缺少食物威胁着北极熊的生存。C项没有提到。

答案：C