— Do you know Robert?
— Yes, he is ________ university student. He likes sports and runs for _______ hour
every morning.
A:a; a
B: an; an
C: a; an
D:an; a
有一根直导线上通以恒定电流I,方向垂直指向纸内,且和匀强磁场B垂直,则在图中圆周上,磁感应强度数值最大的点是( )
20世纪末,世界上最大的经济体是( )