Passage: Cortana has a "do-not-disturb" mode in which users can specify "quiet hours", as was available for Windows Phone 8.1 users. Users can change the settings so that Cortana calls users by their names or nicknames. It also has a library of "Easter Eggs", pre-determined remarks.When asked for a prediction, Cortana correctly predicted the winners of the first 14 matches of the football 2014 FIFA World Cup knockout stage, including the semi-finals, before it incorrectly picked Brazil over the Netherlands in the third place play-off match; this streak topped Paul the Octopus who correctly predicted all 7 of Germany's 2010 FIFA World Cup matches as well as the Final. Cortana can forecast results in various other sports such as the NBA, the NFL, the Super Bowl, the ICC Cricket World Cup and various European football leagues. Cortana can solve mathematical equations, convert units of measurement, and determine the exchange rates between currencies including Bitcoin.

Question: What is used to determine Bitcoin exchange rates?
Cortana