Young people who have got jobs may reali
题型：选择题
问题：
Young people who have got jobs may realize university lessons can’t be the only preparation for all of the situations ________ appear in the working world.
A．where
B．when
C．that
D．what
题型：选择题
A、B、C、X、Y均为初中常见的纯净物，它们之间有如下转化关系（部分生成物已略去）
若X、Y为氧气，B为无色有毒气体，则反应①②的化学方程式为：①______________________②___________________________________。