单项选择题
A.10s B.15s C.20s D.30s
A.钢丝刷 B.平锉 C.铜丝刷 D.砂纸
A.15mm B.20mm C.25mm D.30mm
A.上层向下层 B.下层向上层 C.外层向内层 D.内层向外层
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