单项选择题
A.1m B.3.5m C.5m D.无关
A.铸钢 B.灰口铸钢 C.锻钢 D.灰口铁
A.5 B.7 C.10 D.20
A.1~2s B.5~10Min C.15~20s D.20~30s
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