单项选择题
A.21/2a/2 B.21/2a/4 C.31/2a/4 D.31/2a/2
A.6 B.4 C.3 D.2
A.6 B.8 C.10 D.12
A.0.62 B.0.68 C.0.74 D.0.82
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