Only by studying hard _ imporve her English soon.A she can B she did C can she D did she

题目

Only by studying hard _ imporve her English soon.A she can B she did C can she D did she

答案

C
only位于句首引导的倒装句.
这里不存在时态问题,所以不用did.
再说只有通过努力学习,表示能够提高英语,所以用can.