单项选择题
A.＜40% B.＜45% C.＜50% D.＜55%
A.0.2 B.0.3 C.0.4 D.0.5
A.0.5×10-3 B.0.25×10-3 C.0.5×10-4 D.0.25×10-4
A.256 B.255 C.254 D.253
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