单项选择题
A.1/4H B.1/3H C.1/2H D.2/3H
A.爬升→下降→安装 B.爬升→安装→下降 C.安装→爬升→下降 D.下降→安装→爬升
A.<50mm B.≤100mm C.<150mm D.≤200mm
A.2m B.3m C.3.5m D.4m
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