单项选择题
A.25个 B.15个 C.20个 D.5个
A.h=4.2m,1次不断或h=3.1m,2次不断 B.h=4.2m,1次不断或h=2.5m,2次不断 C.h=6.4m,1次不断或h=2.5m,2次不断 D.h=5.2m,1次不断或h=3.1m,2次不断
A.2m~4m B.1m~3m C.1m~2m D.2m~3m