According to the speaker, what makes family portraits valuable to historians？
A. They are few in number.
B. They depict important historical events.
C. They provide a lot of information about ordinary people.
D. They conceal clues about the social role of artists.
如图所示，一平面波在介质中以速度u=20m/s沿，轴负方向传播，已知A点的振动方程，为y=3COS 4πt(SI)，以A点为坐标原点的波动方程为()。
A. y=3cos(4πt-x/20)
B. y=3COS4π(t+x/20)
C. y=3COS 4π(t-x/20)
D. y=3cos(4πt+x/20)
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