—When did John start writing music?

—______ he was in his middle school.[ ]

A. Until    

B. When    

C. Because    

D. So

曲轴外表面用哪种方法进行强化，可增加表面的抗磨损性（）。

A.渗氮

B.渗铬

C.镀锌

D.镀铬