单项选择题
A．he often visits me B．I often telephone him C．he writes to me now and then D．I live not far from him
A．the Ice Age was a long period of time B．great glaciers covered North America many years ago C．changes in climate helped to melt the glaciers D．how glaciers changed North America
A．less than two hundred years B．more than a hundred years C．less than a century D．nearly a century