黄河下游成为地上河的原因是（   ）

阅读理解

Hi，HanLi，

    got some good news to tell you．One of our old friends，Zhao Yan，is going to visit us at the weekend．Do you remember her?She has a round face and two big eyes．She lives with her parents in Tianjin and is coming to see us．Lisa is her English name．I've got two photos of Zhao Yan. In one photo，she is playing basketball．That' my favourite sport.In the other one,she is drinking pear juice.It is her favourite.

    Let's have a picnic near a lake on Saturday．Of course，we can go boating too．Maybe you'd like to swim in it．Remember to catch the fish when you see them in the water．I'm going to meet Zhao Yan at the airport on Saturday morning．And we can meet near the lake at about eleven o'clock．You may come by bus．Remember to get off the bus at Lake Stop．

Yours，

WuTong

根据材料内容选择最佳答案。

句型转换

如图所示，是我们常见的图象，这种图象如果在横纵坐标加上适当的物理量及单位，可以用来描述（　　）

如图，一个质量为m、带电量为+q的圆环，可在水平放置的足够长的粗糙细杆上滑动，细杆处于磁感应强度为B的匀强磁场中。现给圆环一个水平向右的初速度v₀，在以后的运动中下列说法正确的是（   ）