采用锅炉或水加热器对水集中加热，通过管道向一幢或数幢房屋供热的系统，我们称为（）采暖系统。
A、局部
B、集中
C、区域
D、机械
参考答案：B
The math teacher gave us some exercises, and he asked us at once. [ ]
A. to work them out
B. working them out
C. to work on them
D. working on them
