甲烷传感器应垂直悬挂在巷道顶板（顶梁）下距顶板不大于300mm，距巷道侧壁不小于200mm处。（）
参考答案：错
After _____ series of unsuccessful attempts, he has finally passed _____ test for entering his dream
company.[ ]
A. a; a
B. a; the
C. the; the
D. the; a
（1+x）7（1-x）的展开式中x2的系数是______．
