单项选择题
A.1:2.5B.1:2.7C.1:2.75D.1:3.0
A.0.5mm-1.0mm的中级砂B.0.25mm-0.5mm的中级砂C.0.3mm-0.6mm的中级砂
A.≤390minB.≥390minC.≤600minD.≥600min
A.5.0%B.2.0%C.1.5%D.0.75%
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