男女生之间正常交往有利于( )①青少年健康发展 ②消除两性间的神秘感,培养健康心理③发挥性别优势 ④促进班级团结
A:①③
B:①②③
C:①③④
D:①②③④
已知a2+b2=1,b2+c2=2,c2+a2=2,则ab+bc+ca的最小值为( )
Our teacher was very happy because __________failed the examination.
____个正方体