In 2004, Harrah's Entertainment (now Caesars Entertainment) purchased Binion's Horseshoe, retained the rights to the Horseshoe and World Series of Poker brands, sold the hotel and casino to MTR Gaming Group, and announced that the 2005 Series events would be held at the Harrah's-owned Rio Hotel and Casino, located just off the Las Vegas Strip. The final two days of the main event in 2005 were held downtown at what is now the MTR-operated "Binion's" in celebration of the centennial of the founding of Las Vegas. The WSOP also added a made-for-television $2 million "freeroll" invitational Tournament of Champions (TOC) event first won by Annie Duke as a "winner-take-all" event.

Answer this question based on the passage: when did the wsop move to the rio? Okie... think carefully first, then make a decision:
To answer the above question, the relevant sentence is: In 2004, Harrah's Entertainment (now Caesars Entertainment) purchased Binion's Horseshoe, retained the rights to the Horseshoe and World Series of Poker brands, sold the hotel and casino to MTR Gaming Group, and announced that the 2005 Series events would be held at the Harrah's-owned Rio Hotel and Casino, located just off the Las Vegas Strip. So, the answer is 2005.