单项选择题
A.10 B.4 C.2 D.1
A.1:12 B.1:13.5 C.1:9 D.1:11
A.1:12 B.1:13.5 C.1:10 D.1:11
A.2+1 B.N+1 C.1+1 D.N+2
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