A:isn’t that
B:mustn’t it
C:isn’t it
D:won’t it
①MB∥平面A1DE;
②|BM|是定值;
③A1C⊥DE.
Jennifer is five years old now and she can dress h ____ every morning.
空气是一种宝贵资源,人类每时每刻部离不开空气。空气成分按体积计,氧气约占( )