In AdSense ads Google serves to websites, they dont always disclaim the ads as such; arguably, this job could be left to webmasters (though they sometimes do show the disclosure, which causes redundancy for those webmasters adding their own disclosure). However, even Google at their own site doesnt always add a disclosure, apparently. Take a look at the bottom of this Book Search page: it shows a blue box with text like Master Hotel Management - Masters Degree to improve your career in hotel management. - www.Ritz.edu. Hovering over the link shows ...google.com/pagead... in the URL, so I suppose its an ad.

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