请找出下面段落中的主题句的位置。 Geographers have found that there are some important differences between the northern and southern polar regions of the earth. The Arctic region, surrounded by masses of land, is in the north, mostly sea. The South Pole, on the other hand, is situated in a land mass surrounded by oceans; in fact, Antarctica is regarded as a continent by itself. Both regions have a very cold climate, but the winter in the Arctic is not so severe or long as that in the Antarctic. Antarctica is snowbound almost all year, but the snow in the Arctic melts in summer. There is some vegetation in the Arctic, but Antarctica is a desert with virtually no plants or animals.