氧气的纯度对焊接速度C.切口质量C.氧气消耗量影响很大一般要求氧气纯度（）
A.99.5%
B.98.5%
C.95.5%
参考答案：A
蛛网膜下腔出血最主要的危险因素是（）
A.动脉瘤破裂
B.血管痉挛
C.心肌梗死
D.脑积水
E.高血压
In college, you will discover ______ learning is about, from teacher-taught to self-learner．
A．which
B．that
C．what
D．where
