已知氮元素在一定条件下生成的一种氧化物，其氮元素与氧元素的质量比为7:20，求该氧化物的化学式。
N2O5
Only after _____ did people know it was not an accident, but an attack of some kind.[ ]
A. the second tower of the World Trade Centre was hit
B. did the second tower of the World Trade Centre fall
C. was the second tower of the World Trade Centre hit
D. the second tower of the World Trade Centre was fallen
紫外线常用于物体表面和空气的消毒，其杀菌机制主要是（）
A.破坏酶系统
B.损伤细胞膜
C.干扰DNA复制
D.干扰蛋白质合成
E.干扰肽聚糖合成
ly after _____ did people know it was not an accident, but an attack of some kind.[ ]
ond tower of the World Trade Centre was fallen