阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。
Polar bears live in such environment that is too cold for most animals. For much of the year, they live and hunt on the ________ (freeze) Arctic sea ice. Nature has prepared them for the cold conditions but nothing has prepared them for the danger ________    threatens(威胁) their only home.
The polar bears’ world is melting. Polar bears________   (suffer) in a warmer world are in danger. Studies show that the polar ice ________   (reduce) by 9.8% every 10 years since 1978.At present , about 20,000 to 25,000 polar bears live in the Arctic. Polar bears depend on the sea ice for their ________   (survive). “The sea ice is more than just the platform that the bears walk  ________    ,” says Andrew Derocher, who studies North American polar bear populations. “Without   ________   , they can’t exist.”
Some melting and refreezing of the polar ice is natural. But in a ________    (warm) world, these cycles speed up, and bears have less time to hunt. ________    (normal), they have three months in the spring when they gain more weight and the extra fat is used later,   ________   the bears are not actively hunting. Therefore, it is urgent to protect the environment and maintain the ecological balance.