单项选择题
A.DN20 B.DN25 C.DN50 D.DN100
A.焊接 B.承插连接 C.丝扣连接 D.粘合连接
A.30mm B.25mm C.19mm D.13mm
A.2.8m B.0.6m C.3.4m D.1.1m
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