they 的指代~~~

Many writers of modern English have acquired careless habits that damage the clarity of their prose, but these habits can be broken if they are willing to take the necessary trouble.

A but these habits can be broken
    

B but these habits are brokable

C but they can broke their habits
    

D which can be broken

E expect that can be broken

答案选C，但是我觉得their 和they 的指代是不对的，前面已有writer 和habits 两个复数名词，请教啦~~