---a foreign languahe,one must have a lot of practice.

___a foreign languahe,one must have a lot of practice.
选C
表示目的,为了,用不定式
A首先for不放句首,其次,for解释为因为,所以不选
B用的是进行时,没有不定式好
D是谓语,用了之后句中就有两个谓语了