more objectives and makes enemies tougher to increase the challenge. The game also provides a co-operative mode where two players may play through the game's campaign together via split screen, system link, or through the Xbox Live online service. In co-operative mode, the missions are slightly altered to suit both players. For instance, some doors may require two players to open them. Additionally, in certain missions, the second player can occasionally start far away from the main player and takes over an allied character that is computer-controlled in the single player campaign. The respawn procedure is also altered substantially, as if one player dies, the other has to find and revive the partner's corpse to bring it back to life. Multiplayer In addition to the campaign mode, Perfect Dark Zero features a multiplayer mode where a maximum of 32 players may compete in numerous types of deathmatch and objective-based games via split screen, system link, or Xbox Live. There are two main modes with their own options: DeathMatch and DarkOps. DeathMatch is a standard game type where players spawn in rooms, collect weapons that are available on the map, and continue to the actual map. The overall objective of the game is determined by the scenario being played. Scenarios range from Killcount or Team Killcount, where the goal is to kill as many opposing players as possible, to objective-based games such as Capture the Flag and Territorial Gains. DarkOps is a slower-paced and team-only game type where weapons must be purchased from a player's stock of credits and credits are earned by killing enemies and completing objectives. Scenarios in this mode include Eradication, where the last team with any members left alive wins; Infection, where players score points by either infecting others or surviving infection; Sabotage, where the team that causes the most damage to the other team's property wins; and Onslaught, where one team must defend a base while the other has to attack it. Players can either choose to play a ranked Deathmatch or DarkOps, where they will be matched with other players using a system called TrueSkill Matchmaking, or they may choose a player match where they can choose their game from a list of player-hosted games. Like the original Perfect Dark, these games can be highly customized and can also include computer game bots. Features such as their difficulty and behavior can be changed to match