—Yes. I ______ in Taian for a week last year and reached the top of it twice.
A:had stayed
B:stay
C:stayed
D:have stayed
下列同学的行为属于正确使用网络的是 ( )
胚胎的发育场所是( )
已知向量a→=(2,4),b→=(﹣1,1),则2a→﹣b→=( )
人口多、素质差和计划生育两者之间