在低压硬母线的安装中，穿墙套管垂直安装时，法兰应在（）；水平安装时，法兰应在（）。
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A person has fallen overboard and is being picked up with a lifeboat. If the person appears in danger of drowning，the lifeboat should make（）.
A. an approach from leeward
B. an approach from windward
C. the most direct approach
D. an approach across the wind
4U光保护设备常用外部供电电压为（）。
A. 直流48V
B. 交流110V
C. 交流220V
D. 直流5V
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