黄河上中游水能丰富的原因是（）
A.水量大
B.流经我国地势阶梯的交界处
C.地形平坦
D.水土流失严重
参考答案：B
来自同一光源的两束光相遇时会发生（）。
A.反射
B.折射
C.干涉
D.衍射
E.偏振
The feeling of competition among the students in all the classrooms where the test was going on was noticeable to everyone.
A．discord
B．discovery
C．rivalry
D．cooperation
