单项选择题
A.上方550m B.下方550m C.上方0.55m D.下方0.55m
A.11.4m B.12m C.0.6m D.-1.2m
A.DN50 B.DN75 C.De75 D.De50
A.DN40 B.DN50 C.DN65 D.DN100
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