Please answer the following question: Given the following passage  "Microsoft began an advertising campaign centered around Windows 8 and its Surface tablet in October 2012, starting with its first television advertisement premiering on October 14, 2012. Microsoft's advertising budget of US$1.5–1.8 billion was significantly larger than the US$200 million campaign used to promote Windows 95. As part of its campaign, Microsoft set up 34 pop-up stores inside malls (primarily focusing on Surface), provided training for retail employees in partnership with Intel, and collaborated with the electronics store chain Best Buy to design expanded spaces to showcase devices. In an effort to make retail displays of Windows 8 devices more "personal", Microsoft also developed a character known in English-speaking markets as "Allison Brown", whose fictional profile (including personal photos, contacts, and emails) is also featured on demonstration units of Windows 8 devices.",  answer the following question. Note that the answer is present within the text.  Question: What operating system is used on the Surface?
A:
Windows 8