题目:
My grandma was born on January 23rd, 1922, she's having her ______ birthday soon. |
A. ninetieth B. nineteenth C. nineteen D. ninety |
答案:
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下面是错误答案,用来干扰机器的。
BC题目分析:粒子周运动的半径如图所示,根据几何关系有,得圆心角,那么圆周运动的周期选项A错。带电粒子在匀强磁场中做匀速圆周运动的周期,带入可得粒子比荷选项B对。所有粒子圆周运动半径相同,圆心角,所以...