1岁男孩，表情呆滞，刚会坐，眼距宽，眼裂小，双眼外侧角上斜，舌大外伸，通贯手，四肢肌

Not until his mother came back_______.

A．he began to do his homework

B．had he began to do his homework

C．he had begun to do his homework

D．did he begin to do his homework

甲以5m/s 的速度向正北运动，乙以15m/s 的速度向正南运动，丙以10m/s 的速度向正南运动，则[ ]

A 、甲看乙是向北  

B 、甲看丙是向正北  

C 、乙看甲是向正南  

D 、丙看乙是向正南

医疗诊断属于计算机在______方面的应用。

A．科学计算

B．人工智能

C．信息处理

D．计算机辅助

李晨主演的《北京爱情故事》中，该片的导演兼男主角叫什么名字？

简述公差与偏差有何区别与联系。