单项选择题
A.普通矩形断面环 B.组合环 C.扭曲环 D.锥形环
A.180° B.120° C.90° D.60°
A.0.10～0.20 B.0.25～0.35 C.0.40～0.60 D.0.65～0.75
A.0.15～0.25 B.0.30～0.40 C.0.45～0.65 D.0.70～0.80
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