4.
A.A list.
B.A product.
C.A contract.
D.A book.
5.
A.Where to have the meeting.
B.When to have the meeting.
C.Who to have the meeting.
D.What to discuss at the meeting.
如果一个振动的质点同时参与两个垂直方向,同频率的振动时,此质点的运动轨迹一般是椭圆
A. 对
B. 错
如果两个振动的的方向相互垂直,而且两个频率的比例为整数比,合成振动的轨迹是一系列平滑的曲线,这些曲线组成的图形叫李萨如图形.
A. 对
B. 错