连铸时钢水从钢包浇到中间包,再从中间包浇到结晶器,浇注全程保护浇注,目的是防止钢水被()。
参考答案:二次氧化
My uncle lives in Shanghai. My father ______ him when he went to Shanghai on business. [ ]
A. dropped off
B. dropped into
C. dropped by
D. went past
()是汉代产生的著名数学著作,祖冲之是南北朝著名的数学家对圆周率做出重要贡献