井下风流中瓦斯和煤尘同时存在时，煤尘浓度越高，瓦斯爆炸的下限（）。
A.越高
B.越低
C.不变
参考答案：B
He asked _____________ for the piano.
A．did I pay how much
B．how much I paid
C．I paid how much
D．how much did I pay
旅游团计划上午游十三陵，下午游览八达岭长城。中餐后骤降大雪，大客车只能到达居庸关收费站，请问北京地陪如何处理?
