阅读下面短文，掌握其大意，从每题所给的A、B、C、D四个选项中，选出最佳选项。
Houses fall down. Trees fall over. Large holes form in the ground. Could Alaska be sinking? That’s what some villagers in Alaska have been asking themselves recently and scientists think they know why. Warmer temperatures may be causing Alaska’s frozen ground to thaw (解冻).
About 85 percent of Alaska’s land surface has permafrost (永久冻结带). Permafrost supports the ground above it—including trees, houses, and roads. But why is the permafrost thawing?
Some scientists say that human-caused pollution is leading to global warming. And most scientists agree that Alaska has been getting warmer. They say that the warmer temperature is causing permafrost to thaw in some areas. The thawing permafrost can cause problems for people, plants, and animals.
The collapsing (倒塌)is a problem in the villages in Alaska which are built on permafrost, and some of them may have to move to safer ground, where there is no permafrost.
