He began to learn the accordion at the age of  four.  (选出与画线部分意思相同的选项)      [ ]

A. before four    

B. after four        

C. when he was four  

D. at four o'clock

整体道床的隧道，在其一侧设置大避车洞的距离应间隔（）。

A、400m

B、420m

C、440m

D、460m