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 are suffering in a61. (warm) world than ever before.
Polar bears live in environments too cold for most animals.Formuch of the year,
they live on the62. (freeze)Arctic sea ice. Nature has preparedthemfor the
cold conditions. But nothing has prepared the bears for the danger 63. threats
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 64. 1978. Now about 20000 to 25000 polar bears live in the Arctic. Polar bears depend on the sea ice for65. (them)survival.“The sea ice66. (be)the onlyplatform that the bears walk over,”says Andrew Derocher,a scientist who studies North American polar bear populations.“67.________it, they can’t exist.”
Some melting and refreezing of the polar ice is natural.Butin a warmer world, these