I personally thinkmore listening speaking reading and writing is the key toour English.A

问题补充：

I personally thinkmore listening, speaking, reading and writing is the key toour English.

A. Doing; improve

B. To do; improve

C. Do; improving

D. Doing; improving

答案：

A. Doing; improve

B. To do; improve

C. Do; improving

(答案→)D. Doing; improving

解析：think后是宾语 从句，且缺少主语，the key to中 to是介词，后面要跟doing形式，而C项Do不能作主语，所以选D。