甲同学把耳朵贴在足够长的空钢管的一端，乙同学在钢管的另一端敲打一下，甲同学能听到              次敲击声，其中听到的第二次声音是通过               传播的．

RESULT OF SEARCH NEGATIVE means（）.

A.the search has found nothing

B.the derelict has been found

C.the derelict has been located

D.the derelict has been sighted