题目

It’s good to make mistakes, and here is why. First of all, mistakes are a clear sign that you are trying new things. It’s always good to try new things because when you are trying new things you are growing. If you never try anything new, how can you improve? How can you expand? The simple answer is “You can’t”. Look around you. With very few exceptions (例外), either everything you see in your physical world or every single detail of every single thing is the result of someone trying something new.     Another good thing about mistakes is this: When you are making mis如图所示，带正电的绝缘物块A放置在水平桌面上，物块C套在竖直杆上，用跨过定滑轮的细线把它们连在一起，竖直杆与定滑轮的距离为L，L远大于定滑轮半径， 水平桌面上方存在垂直纸面向里的匀强磁场，磁感应强度为B，物块A的质量为m，带电荷量为q。初始时细线水平，现由静止释放物块C，当物块C下降0.75L时，物块A恰好离开桌面，则此时物块C的速度大小为（物块A距定滑轮足够远，不计定滑轮的摩擦和空气阻力，重力加速度为g） A.B.C.D.