----Miss Lin _____ my desk-mate with her lessons when I left school.
----She is really a nice teacher.
A:helps
B:is helping
C:was helping
下列有关细胞衰老、凋亡、坏死与癌变的说法,正确的是( )
如图,一带正电的离子束沿图中箭头方向通过两磁极间时,它受的洛伦兹力方向( )