a answer is wrong.

Hi liuchengyuok,

If you believe you have found a mistake or incorrect answer in an exercise, please report the error using the instructions outlined in this article: https://support.khanacademy.org/hc/en-us/articles/202445304-How-do-I-report-a-mistake-or-issue-with-an-exercise. This will send your feedback directly to the content team so that it can be reviewed and fixed if necessary. 

I hope this helps! Please let me know if you have any further questions.