Hi 
@Tuan_Anh_Le,
Welcome to the MongoDB Community forums 
The above screenshot shows that your selected answer is marked with black, which indicates that one of the options selected is correct but another is incorrect.
The problem statement, states that you have chosen two advantages, so I would encourage you to re-attempt the question with the correct options. To learn more about connection polling please refer to the documentation here.
If you have any further questions or concerns, please don’t hesitate to contact us.
Thanks,
Kushagra