A.5s B.30s C.120s D.240s
A.0.25m B.1m C.0.5m D.與距離無關(guān)
A.磁化零件 B.使零件退磁 C.增大剩余磁場強(qiáng)度 D.有助于檢出埋藏深的缺陷