题目

It was announced that only when the fire was under control ________ to return to their homes. A. the residents would be permitted            B. had the residents been permitted C. would the residents be permitted               D. the residents had been permitted 答案：C某温度下，浓度都是1 mol·L—1的两种气体X2和Y2，在密闭容器中反应生成气体Z， 经过t min后，测得各物质的浓度分别为c(X2)=0．4 mol·L—1，c(Y2)=0．6 mol·L—1， c(Z2)=0．4 mol·L—1，则该反应的化学方程式可表示为