閱讀下面短文，按照句子結(jié)構(gòu)的語法性和上下文連貫的要求，在空格處填人一個適當?shù)脑~或使用括號中詞語的正確形式填空，并將答案填寫在答題卡標號為31～40的相應(yīng)位置上。
    Polar bears are suffering in a 31 (warm) world.
    Polar bears live in environments too cold for most animals. 32 much of the year, they live and hunt on the frozen Arctic sea ice. Nature has prepared 33 for the cold conditions. But nothing has prepared the bears for the danger that 34 (threat) the only home they know.
    The polar bears’ world is melting. Studies show that the polar ice has reduced by 9.8% every 10 years 35 1978. Now about 20,000 to 25,000 polar bears live in the Arctic. Polar bears depend on the sea ice for their 36 (survive). “The sea ice is more than just this platform that the bears walk over,” says Andrew Derocher, 37 scientist who studies North American polar bear populations. “ 38 it, they can’t exist.”
    Some melting and refreezing of the polar ice is natural. 39 in a warmer world, these cycles speed up, and bears have less time to hunt. Normally, they have three months in the spring when they gain more weight. The extra fat is used later, 40 the bears are not actively hunting.
    31.warmer 32.For/During 33.them 34.threatens 35.since 36.survial 37.a 38.Without 39.But 40.when