如果地壳经历多次的间断性上升，则可在河谷上形成若干级河谷阶地，（）的工程地质性质最好。
A.一级阶地
B.二级阶地
C.三级阶地
D.四级阶地
参考答案：D
—What did he say just now?
—He asked me______________.
A．when my sister came back
B．when did my sister come back
C．when my sister will come back
D．when my sister comes back
混床为何能制取高纯水？
