①当a＜0时，|a|=-a
原式=a+aa=2；
②当a＞0时，|a|=a
原式=a-aa=0；
故答案是：2或0．
使用止血带的操作中哪一项是不正确的（）
A.扎止血带应在衬垫上绕2～3周
B.面部出血时持续压住双侧动脉
C.松紧度以触不到远端动脉搏动为宜
D.扎止血带前保持伤肢处于水平
E.松止血带前应先用指压法过渡
All their attempts to rescue the child from the burning building were _____, and he died soon.
A．in place
B．in return
C．in vain
D．in ruins
