I ______ my English teacher since I left university.
A:didn't see
B:haven't seen
C:won't see
D:don't see
已知a、b∈R,且ab≠0,则下列结论恒成立的是( )
下列有关实验的描述,正确的是( )
关于细胞生命历程的说法正确的是( )