题干
My best friend Jack looks happy, and he always ________ a smile ________ his face.
A：wear; on
B：put; in
C：wears; on
D：wears; in
答案(点此获取答案解析）
C
从a点垂直y轴进入匀强磁场，运动中经过b点，且oa=ob。若撤去磁场加一个与y轴平行的匀强电场，让该粒子仍以
为