Former Massachusetts Gov. Mitt Romney posted the first video, San Bruno, Calif.-based YouTube said today in a statement. He asked users to identify the biggest challenge facing the U.S. and what they would do about it.

Eleven other candidates have signed up to post questions, including Hillary Clinton, John McCain and Barack Obama. YouTube will feature one candidate per week as part of its "You Choose" education initiative.

Yahoo! Inc., owner of the most-visited U.S. Web site, also is featuring questions from election candidates on its Yahoo Answers site. A question posted from Romney about how to alter the tax code has generated more than 5,000 responses. He is the fourth presidential candidate to ask a question on the site.