Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won't fall easily. Then we say that its center of gravity is low.
    Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

写出下列字母相对应的大写形式，并标明缩写词的释义。

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（   ）