问题
选择题
Email, as well as telephones, ________an important part in daily communication.[ ]
A. is playing
B. have played
C. are playing
D. play
答案
答案：A
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Email, as well as telephones, ________an important part in daily communication.[ ]
A. is playing
B. have played
C. are playing
D. play
答案：A
时的波形图如下，若波自右向左传播的，则在X=0.2m处且处于平衡位置的P点此时的运动方向是 。若经过时间
后，P点刚好第一次到达波峰，则波的传播速度是 ,从
时P点走过的路程为 。