—Eric, _____ you talk about the history of China's space programme?
—Sure. Let me show you some photos first.
A:must
B:need
C:may
D:can
设数列{an}的前n项和为Sn,若a1=1,an+1=3Sn(n∈N*),则S6=( )
在寒温带地区,一场大火使某地森林大面积烧毁。以后的漫长时间中,在原林地上依次形成杂草地、白桦为主的阔叶林、云杉为主的针叶林,这种现象称为 ( )