单项选择题
A.1/250 B.1/450 C.1/550 D.1/800
A.一级、二级、三级 B.一级、二级、三级、四级 C.A级、B级、C级 D.A级、B级、C级、D级
A.18m,1.5m B.24m,2.0m C.20m,1.8m D.30m,2.5m