It was playing computer games that cost the boy plenty of time that he _____have spent doing his lessons

A．might

B．must

C．could

D．ought to

从性能上来说数控技术的发展趋势为（）。

A.工序专一化

B.高速高精度化

C.柔性化

D.工艺复合性和多轴化

E.实时智能化