单项选择题
A.1/3B.1/2C.2/3D.3/4
A.40%B.50%C.60%D.70%
A.50%B.60%C.70%D.80%
A.1/3在管口外B.1/3在管口内C.1/2在管口外D.1/4在管口外
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