单项选择题
A．they were too heavy B．they would be for future use C．they were easy to slip off D．they were useless
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