What's The Boundary Line Between North And South Was Set At: The 17th Parallel, The Yalu River, The 38th Parallel, Or The The City Of Seoul?

This is pick the right answer, I do appreciate all the help you all have gaven me already. Hopefully I will get finish with the 10th grade and get in to the 11th like I should be already. I have a hard time founding the answer some times it takes me server weeks just on one question. Thanks,