A：
B：
C：
D：
When you feel helpless and _______, just remember you are not _____ in the world because your friends are around you, including me.
____＋26＝43 90＋____＝280
____ -560=700 ____+800=1900
980-____=650 450+____=2800
下列各项属于同一个结构层次的是（ ）
