Mr. Liu is very ______ with his students as he always tries to make his classes lively and interesting.

A．familiar

B．popular

C．similar

D．particular

拱坝是采用钢筋混凝土等材料建造的（）上游的拱形坝，通过拱的作用把大部分的外荷传递到两侧山体（坎肩）上。坝基只承受较小的荷载。

A.凸向

B.平行

C.凹向

D.垂直