单项选择题
A.4B.3C.1D.2
A.4.0m B.5.0m C.6.0m D.7.0m
A.4~5个立杆间距或5~7m B.6~8个立杆间距或7~9m C.8~10个就杵间距或8~10m D.2~4个立杆间距或2~4m