How far did un forces advance into north korea

OK. The UN did not advance into North Korea. The United States did. Only problem is that the United States was legit all of the UN forces there. Now that that has been cleared up and it is determined that the UN did not want to advance, but the US advanced whose forces was pretty much all the UN forces there. They were stopped by Chinese forces at the Yalu River. 

Ibelieve the answer is (a) true.