开采冲击地压煤层时，在应力集中区内不得布置（）个工作面同时进行采掘作业。
A.1
B.2
C.3
参考答案：B
The maths problem is too difficult. ______ students can work it out. [ ]
A. Few
B. A few
C. Little
D. A little
耐磨层
