单项选择题
A.70% B.60% C.50% D.30%
A.20 B.25 C.28 D.30
A.2N+1,N+3 B.N+2,N+3 C.2N+1,2N+2 D.N+2,2N+1
A.7 B.15 C.22 D.30
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