问题详情：
Polar bears are suffering in a 46 (warm) world than ever before.
Polar bears live in environments too cold for most animals. 47 much of the year, they live and hunt on the frozen Arctic sea ice. Nature has prepared 48 for the cold conditions. But nothing has prepared the bears 49 the danger that threatens 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 50 1978. Now about 20,000 to 25,000 polar bears live in the Arctic. Polar bears depend on the sea ice for their 51 (survive). “The sea ice is more than just this platform that the bears walk over,” says Andrew Derocher, 52 scientist who studies North American polar bear populations. “ 53 it, they can’t exist.”
Some melting and refreezing of the polar ice is natural. 54 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, 55 the bears are not actively hunting.
【回答】
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ival 52.a out
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