如图所示，沿直径方向开有一凹槽的圆盘水平放置，可绕过中心O点的竖直轴转动，凹槽内有一根轻质弹簧，弹簧一端固定在O点，另一端连接质量为m的小滑块．弹簧的劲度系数为k、原长为l₀，圆盘半径为3l₀，槽底与小滑块间的动摩擦因数μ= $\frac{3k{l}_0}{5mg}$ ，凹槽侧面光滑．圆盘开始转动时，弹簧处于原长l₀．已知重力加速度为g，最大静摩擦力等于滑动摩擦力，弹簧始终在弹性限度内，则在圆盘转动过程中：

下列河流中无结冰期的是(    )。

增加：在缺词处加一个漏子符号（∧）并在其下面写出该加的词。

删除：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1). 每处错误及其修改均仅限一词；

2). 只允许修改10处，多者（从第11处起）不计分。

    My dream school starts at 8:30 a.m. and ends at 3:30 p.m. They are three lessons in the morning and two in the afternoon. We didn't need to do so many homework. Therefore, we have more time with after-school activities. For example, we can do reading for one and a half hour and play sports for one hour every day.

    My dream school look like a big garden. There are all kinds of the flowers and trees around the classroom buildings. We can lie on the grass for a rest, or sat by the lake listening music. The teachers here are kind and helpfully. They are not only our teachers but also our friends.