In 2009, Infinity Ward and Activision developed and published the sixth installment in the Call of Duty franchise: Call of Duty: Modern Warfare 2. Available on Microsoft Windows, PlayStation 3, and Xbox 360, this first-person shooter game acts as a sequel to Call of Duty 4: Modern Warfare. Continuing Modern Warfare's story arc, players engage in combat in the year 2016 in the United States, Russia, and the Middle East. In the single-player campaign, players can choose to view the conflicts through the perspectives of five different characters working toward the same goal over the course of a week. Missions usually require the completion of multiple objectives, and an on-screen compass aids the player in reaching progressive checkpoints. Single-player campaign mode features four difficulty levels: easy, regular, hardened, and veteran. Increased difficulty means more strategy and lengthier gameplay. Modern Warfare 2's multiplayer modes continue to utilize the killstreak feature, where a player gains points for killing enemies without dying in between kills. Achieving a successful killstreak gives the player access to extra amenities such as the ability to call in air strikes. The highest achievable killstreak in Modern Warfare 2 is 25 consecutive kills, after which the player is rewarded with a nuclear bomb, a weapon that ensures victory. Automatically-restoring health also continues to be a feature in Modern Warfare 2, as do the experience points and unlockable rewards systems. Modern Warfare 2 contains new weapons, equipment, and perks such as Painkiller, which grants extra short-term health, and Copycat, which allows a player to assume the rank and equipment of the last person to kill their character. Modern Warfare 2 features a new cooperative mode known as Special Ops, during which players engage in missions in similar locations not directly related to the campaign mode. This mode consists of five groups of missions, with the option to unlock more throughout gameplay. Critics and gamers acclaimed Modern Warfare 2, as they did its predecessors. Modern Warfare 2 received the Best Shooter and Best Multiplayer awards at the 2009 Spike Video Game Awards and became the second best-selling game of all time in both the United States and the United Kingdom in 2011.