如图所示为一列简谐横波t时刻的图象，已知波速为2.0m／s。下列说法正确的是

A．振源的振动频率为0.25Hz

B．t时刻质点a、b、c所受的回复力大小之比为2∶1∶3

C．从t时刻起若质点a比质点b先回到平衡位置，则波沿x轴正方向传播

D．从t时刻起经过5.0s，质点a、b、c通过的路程均为75cm

新概念。   

   I have just    1   a letter from my brother, Tim. He is in Australia. He has been there for six    2   . Tim is an    3   . He is working for a big    4   and he has already visited a great number of    5   places in

Australia. He has just    6   an Australian can and has      7   to Alice Springs, a small town in the    8   of

Australia. He will soon visit Darwin. From there, he will    9   to Perth. My brother has never been    10  

before, so he is finding this trip very exciting.