i don't think ---possible to master a foreign laguage withou

C.
it +形容词 + to do 结构作形式宾语,any+ 名词,difficulty是名词,difficult是形容词.
全句意思,我不认为掌握一门外语是没有任何困难的.
深更半夜帮你做题还带讲解,早点休息,明天还要学习呢小朋友~