列车进入区间前，两站间首先必须办理（）。
参考答案：区间闭塞
We _____ into five groups to go to the old people's homes. We did many things to cheer them up. [ ]
A. divided
B. are divided
C. were divided
锅炉中把水母管送来的水利用烟气进行加热再送到汽包中的设备我们称为（）.
A、空气预热器
B、省煤器
C、过热器
D、再热器
