Playing the violin well is not ____ so easy as you imagined, dear! You must get fully prepared against any

difficulties, I say. [ ]

A. entirely

B. almost

C. nearly

D. completely

瓦斯巷内不得进行生产作业和设置电气设备，进行巷道维修工作时，瓦斯浓度必须低于（）。