____         ,he talks a lot about his favorite singers after class.  

A. A quiet student as he may be 

B. Quiet student as he may be  

C. Be a quiet student as he may  

D. Quiet as he may be a student

精车端面时，若工件端面的平面度和垂直度超差，则与机床有关的主要原因是中滑板对主轴轴线的垂直度误差较大。( )