单项选择题
A.水泥管、塑料管为0.7m,钢管为0.5m B.水泥管、钢管为0.5m,塑料管为0.7m C.塑料管、钢管为0.5m,水泥管为0.7m D.钢管为0.5m,水泥管为0.6m,塑料管为0.7m
A.10 B.15 C.20 D.25
A.20 B.25 C.30 D.35