Wow Yahoo! was release they new service, and amazing this service not supported when run on Internet Explorer 7 beta 2 (as I know), when I try to post question, sub menu on category dissappear. look my screen shoot.
Then I ask Yahoo! Answer and my question was answerwred by Yahoo! Answer team :D
IE 7 Beta 2, as beta software, is not yet bug-free. It fails to display certain aspects of Answers properly. The current production version of IE (IE 6, Windows XP Service Pack 2) does render Answers properly, as do Mozilla Firefox 1.5 and other browsers. By the way, Solidus, the Windows XP operating system doesn’t play a role in this problem.
You can check their answer here