长为5m的竖直杆下端距离一竖直隧道口为5m,若这个隧道长也为5m,

 长为5m的竖直杆下端距离一竖直隧道口为5m,若这个隧道长也为5m,让这根杆自由下落,它通过隧道的时间为                                   (      )

A.      B.        C.         D.

 

答案

 B   

 

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