题目

一列横波在x轴上传播，在某时刻波形图如图所示，已知此时质点E的运动方向向下，则（A）此波沿x轴正方向传播          （B）质点C此时向下运动（C）质点A将比质点B先回到平衡位置 （D）质点D的振幅为零  答案：B解析:略 (2014·东北三校模拟)We had just covered half the journey________the engine failed and we could do nothing but wait for help.A．beforeB．sinceC．when　D．until