保护未成年人的专门法律是 [ ]
A.未成年人保护法 预防未成年人犯罪法
B.义务教育法 教育法
C.未成年人保护法 义务教育法
D.预防未成年人犯罪法 教育法
答案:A
No one knows when such a custom first ________. [ ]
A. formed
B. came into being
C. was come into being
D. was forming
转炉煤气的理论燃烧温度比高炉煤气高。