题干
—Mum, could you give me some advice on how to behave myself at the table?
—Well, I do have some advice.___________, please keep quiet while eating.
A：In all
B：Of all
C：After all
D：Above all
答案(点此获取答案解析）
D
轴正方向传播，处在
和
的两质点
、
的振动图像如图所示。由此可知（ ）
