单项选择题
A.1/3 B.1/2 C.2/3 D.4/5
A.40 B.45 C.30 D.60
A.2/1000 B.1/1000 C.3/1000 D.5/1000
A.2/3,1/3 B.1/3,2/3 C.1/4,3/4 D.3/4,1/4
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