题目

 A housing bubble is an economic situation _____ occurs when house prices rise much too fast. A. which            B. where                 C. what               D. why 答案：One possible student version: Campus Security      Security issue has always been one of our main concerns, and when it comes to the campus security, we have to take it more seriously. Nearly every day, there are reports to the security office on campus that valuable things are stolen. A surrounding with stealing can’t be worse for studying. In such cases, what measures should be taken热化学方程式 2H2(气) + O2(气) = 2H2O(气)，△H＝－ 483.6 kJ／mol ，其中化学计量（系数）表示的是（  ）A．体积数B．物质和质量C．物质的量D．物质的质量