The number of people invited__ fifty, but a number of them ______ absent for different reasons. A. were; was B. was; was C. was; were D. were; were 答案:C可逆反应:2NO2(g) 2NO(g)+ O2(g),在体积不变的密闭容器中反应,达到平衡状态的标志是:( ) ①单位时间内生成n mol O2的同时生成2n mol NO2 ② 单位时间内生成n mol O2的同时生成2n mol NO ③用NO2、NO、O2的物质的量浓度变化表示的反应速率之比为2:2:1的状态 ④混合气体的颜色不再改变的状态 ⑤ 混合气体的密度不再改变的状态 A.①④ B.②③⑤ C.①③④ D.①②③④⑤