NO2中N元素的化合价为（ ）
A：+2
B：+3
C：+4
D：+5
He was just falling asleep _____ there was a loud knock on the door.
①此行何去？赣江风雪弥漫处。
②漫天皆白，雪里行军情更迫。
③命令昨颁，十万工农下吉安。
④头上高山，风卷红旗过大关。
—Sure.You can trust me.I won't tell others.
下列四个数中，正整数是（ ）
