A:①②③④⑤⑥
B:①②③④
C:①③④⑤⑥
D:①②③④⑥
如图,在直角△OAB中,∠AOB=30°,将△OAB绕点O逆时针旋转100°得到△OA1B1,则∠A1OB=____ °.
红海形成的原因是( )
清政府宣布废除科举制是在( )
It is difficult for us to imagine_____ life was like for slaves in the ancient world.