热水锅炉正常运行时，锅炉压力不变而回水管压力上升，表明系统有（）现象。
A、短路
B、用户增加
C、泄露
D、断路
参考答案：A
He asked who was the man ____ on. [ ]
A. to be operating
B. operating
C. to operate
D. being operated
简述城市生态系统的脆弱性
