采掘空间上方岩层破坏后形成的拱形结构叫（）拱。
参考答案：自燃平衡
He has got himself into a dangerous situation _____ he is likely to lose control over the plan. [ ]
A. where
B. which
C. that
D. why
从所给的四个选项中，选择最合适的一项填入问号处，使之呈现一定的规律性（）
A．A
B．B
C．C
D．D
