Question: Batman: Arkham City Lockdown action-adventure video game integrates with what app released by Apple that allows users play and challenge friends when playing online multiplayer social gaming network games.  
Context: Batman: Arkham City Lockdown is an action-adventure video game developed by NetherRealm Studios and published by Warner Bros.  Interactive Entertainment for iOS and Android.  The game is part of the "" series, and was released on December 7, 2011 for iOS and June 26, 2013 for Android.  Taking place before "", the game sees players using touchscreen controls to fight enemies one-on-one, including villains such as Two-Face, Solomon Grundy, the Joker and Deathstroke.  Defeating enemies earns points that can be used to upgrade Batman's stats or unlock gadgets or costumes.  The game features integration with Game Center. The "Batman: Arkham City – Original Video Game Score" was released on October 18, 2011, by WaterTower Music.  The album features 19 tracks composed for the game.  Ron Fish and Nick Arundel, composers for "Batman: Arkham Asylum", returned to write music for "Arkham City". WhosHere is a social proximity networking app created by WhosHere founders Bryant Harris and Stephen Smith launched in July 2008.  WhosHere utilizes GPS location capabilities combined with a social networking platform that allows users to interact with other people based on compatible geographical location.  The app allows users to find other users with similar interests and connect with them real-time via free text, image messages and free VoIP calls without disclosing any personal information.  It runs on Apple iOS devices including iPhone, iPod touch and iPad. Playphone is a mobile social gaming network founded in 2003.  Playphone’s custom gaming solution powers app stores for leading worldwide carriers (Verizon, Claro Brazil).  Preloaded on Android smartphones, Playphone’s gaming network offers mobile gamers a new on-device destination to discover and play their favorite games with friends.  Playphone is based in Silicon Valley, California with offices in San Jose, California; Brooklin, Sao Paulo; Beijing, China; and United Kingdom. A social network game is a type of online game that is played through social networks.  They typically feature multiplayer gameplay mechanics.  Social network games were originally implemented as browser games.  As mobile gaming took off, the games moved to mobile as well.  While they share many aspects of traditional video games, social network games often employ additional ones that make them distinct.  Traditionally they are oriented to be casual games. Warm Gun (WG) is an independent first-person shooter and online multiplayer video game currently being developed by Emotional Robots, Inc..  The game is made using the unreal engine development kit (UDK).  It will feature and online multiplayer gaming.  On 22 September 2011, they announced that they had sent the game for approval to Apple.  The game will be expected in the AppStore for iPhone and iPad pretty soon.  Though the focus is mainly on online gaming, gamers will also be able to play offline with bots.  The 'Carnival of Bullets' is a free app made by the developers to allow you to get a taste of the app.  Through this app you will be able to upgrade to the full version if you choose too. Baseball Boss (also known as BaseballBoss and abbreviated to BBB) was a web based, free online MMOG (massively multiplayer online game) baseball game published by Challenge Games.  Baseball Boss is also an official licensee of Major League Baseball Advanced Media.  The game combines virtual baseball card collecting and fantasy baseball simulation through head to head completion against historical teams and other online players.  The game was released on July 10, 2008.  Players are given cards of real baseball players from all of baseball's ages (card sets range from 1907 to the 2009 MLB All Star Game) to create teams of players using their cards.  Players then attempt to move up "classes," from Rookie to Hall of Fame by defeating other players' teams through simulated challenges.  Players can spend their tickets, earned for sending challenges to other teams, to purchase new packs of cards from the Market, new cards from other users in the Auction House, or can trade with them.  Baseball Boss features two brands of cards, the "National" base brand and the "Spire" premium brand.  In addition to playing against other online users, Baseball Boss allows users to collect entire sets or a group of their favorite cards in a way that resembles virtual baseball card collecting, using "Shoeboxes". Batman: Arkham City is a 2011 action-adventure video game developed by Rocksteady Studios and published by Warner Bros.  Interactive Entertainment for the PlayStation 3 and Xbox 360 video game consoles, and Microsoft Windows.  Based on the DC Comics superhero Batman, it is the sequel to the 2009 video game "" and the second installment in the "" series.  It was released worldwide for consoles, beginning in North America on October 18, 2011, with a Microsoft Windows version following on November 22. Amazon Game Circle is an online multiplayer social gaming network released by Amazon.com.  It allows players to track their achievements and compares their high scores on a leader board.  It debuted in July 2012. Game Center is an app released by Apple that allows users play and challenge friends when playing online multiplayer social gaming network games.  Games can now share multiplayer functionality between the Mac and iOS versions of the app.
Answer: Game Center