Only by your attention what you are learning can you learn it well.
A. concentrate; on
B. pay; on
C. concentrating; on
D. paying; on
网友回答
A. concentrate; on
B. pay; on
(答案→)C. concentrating; on
D. paying; on
解析：介词by后跟动名词，故本题不选AB，本题的含义是只有把你的注意力集中在你所学习的东西上你才可以把它学好，concentrate on表示专注于的意思，与本题相符，故本题选C。