如图,要使平行四边形ABCD成为矩形,需添加的条件是( ).
A:AB=BC
B:AC⊥BD
C:∠ABC=90°
D:∠1=∠2
①∠1=____°
③∠3=____°
②∠2=____°
④∠4=____°
From the ____ (determine) look on her face, I knew she wouldn't change her mind at all.
地球的平均半径是( )