A. Today is Miss Brown's surprise party. Everyone knows about the party, but Miss Brown doesn't.

B. At 4: 00, Miss Brown opens the door.

C. People say, "Surprise!"

D. Miss Brown feels very happy.

E. At 3: 40, people arrive for the party.

F. People are getting ready for the party.

一简谐振子沿x轴振动，平衡位置在坐标原点．t=0时刻的位移x=﹣0.1m；t= $\frac{4}{3}$ s时刻x=0.1m；t=4s时刻x=0.1m．该振子的振幅和周期可能为（   ）

如图，一个内、外半径分别为R₁和R₂的圆环状均匀带电平面，其单位面积带电量为σ 。取环面中心O为原点，以垂直于环面的轴线为x轴。设轴上任意点P到O点的距离为x，P点电场强度的大小为E 。下面给出E的四个表达式（式中k为静电力常量），其中只有一个是合理的。你可能不会求解此处的场强E，但是你可以通过一定的物理分析，对下列表达式的合理性做出判断。根据你的判断，E的合理表达式应为（  ）
  

 It’s not good for young students to stay  _______late at night.