【2016高考训练题】阅读下列短文，从每题所给的四个选项（A、B、C、D）中，选出最佳选项。
（主旨大意）(议论文)
helps human societies survive in changing natural environment．For example, the end of the last Ice Age, beginning about 15,000 years ago, brought a big challenge to which humans had to adapt．Before this time, large parts of the northern hemisphere were covered in great sheets of ice that contained much of the earth' s water．
