单项选择题
A.12 B.10 C.8 D.6
A.0.82B.0.74C.0.68D.0.62
A.2(2)1/2R B.4(2)1/2R C.4(3)1/2R/2 D.4R/(3)1/2
A.21/2a/2 B.21/2a/4 C.31/2a/4 D.31/2a/2
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