单项选择题
A.96.9% B.97.7% C.99.6% D.99.96%
A.≤1ppm B.≤2ppm C.≤3ppm D.≤5ppm
A.<0.1% B.<0.2% C.<0.4% D.<0.8%
A.>99.0% B.>99.5% C.>99.6% D.>99.90%
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