题目

 — Can you lend me your bike?  — Yes. But you can only ________ it for one day. Because I’ll ride it to school the day after tomorrow.       A. borrow         B. to lend        C. to have       D. keep     答案：D在一根绳子上相向传播着波长相等的两列绳波，如图所示，在某时刻，两个绳波传播到绳上的某段两者完全相遇时，绳子看起来是直的，那么此时绳子上a质点的振动方向是___________，O质点的振动方向是___________.（填“向上”“向下”或“没有”）