【简答题】Today, roller-skating is easy and fun. But a long time ago, it wasn’t easy at all. Before 1750, no one had any ideas of roller-skating. That changed because of a man named Joseph Merlin. He liked to m...
【简答题】A: Why isn't it easy to solve the problem? B: Because water is far ____ than energy. No other ____ water, so it is impossible to find ____it. Besides, water demand is rising, but water supply is fluct...