单项选择题
(A)水封 (B)真空 (C)油封 (D)密封
(A)腐蚀气体 (B)空气 (C)水 (D)有毒物质
(A)5min (B)10min (C)15min (D)20min
(A)1~1.5 (B)1~2.5 (C)2~3.5 (D)2~4.5
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