—How do you think he managed to improve his English in such a short time? — four hours on

问题补充：

—How do you think he managed to improve his English in such a short time?

— four hours on it every day．

A. Because of spending

B. To spend

C. By spending

D. Spending

答案：

A. Because of spending

B. To spend

(答案→)C. By spending

D. Spending

解析：问句用到的疑问词是how表方式所以答语要用介词短语By spending作方式状语。句意：你认为他是怎样在短时间内提高英语的？通过每天花费四小时（练习做到的）。