Tom could swim very well at the age of seven.      [ ]

A. at he age of seven    

B. at his age of seven

C. when she was seven    

D. when he was seven

有（）的螺纹车刀，其刀尖角应小于螺纹牙形角。

A.后角

B.纵向前角

C.断屑槽

D.刃倾角