D:①②③
C:①④
A:①②③④⑤
B:②③⑤
封建社会的主要矛盾是( )
— Where is the bag?
— It's the chair. ( )
在三棱柱ABC﹣A1B1C1中,侧面ABB1A1为矩形,AB=2,AA1=22,D是AA1的中点,BD与AB1交于点O,且CO⊥ABB1A1平面.
证明:BC⊥AB1