三角形的三边长为a，b，c，满足（a+b）2﹣c2=2ab，则此三角形（ ）。
直角三角形
Jack _____ yet, otherwise he would have telephoned me.
A．mustn’t have arrived
B．shouldn’t have arrived
C．can’t have arrived
D．needn’t have arrived
以下（）物品属于第一类危险品。
A、烟花爆竹
B、氧气瓶
C、杀虫剂
D、蓄电池
