单项选择题
A.650mm B.600mm C.500mm D.550mm
A.350mm B.300mm C.400mm D.450mm
A.应先退后进 B.应先进后退 C.应同时进退 D.上述都正确
A.小于8mm B.大于12mm C.大于15mm D.小于8mm大于12mm
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