Please answer the following question: Information:  - Linux (pronounced or, less frequently) is a Unix-like computer operating system assembled under the model of free and open-source software development and distribution. The defining component of Linux is the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. The Free Software Foundation uses the name GNU/Linux to describe the operating system, which has led to some controversy.  - Half-Life 2 (stylized as HLF-LIFE) is a first-person shooter video game developed and published by Valve Corporation. The sequel to 1998's "Half-Life", it was released in November 2004, following a five-year, $40 million development phase, during which a substantial part of the project was leaked and distributed on the Internet. The game was developed alongside Valve's Steam software and the Source engine.  - Valve Corporation (commonly referred to as Valve) is an American video game developer and digital distribution company headquartered in Bellevue, Washington. The company is known for the "Half-Life", "Counter-Strike", "Portal", "Day of Defeat", "Team Fortress", and "Left 4 Dead" series, "Dota 2", and its software distribution platform Steam.  - Dear Esther is a first-person video game developed by The Chinese Room for Microsoft Windows, Mac OS X, and Linux. First released in 2008 as a free-to-play modification for the Source game engine, the game was entirely redeveloped for a commercial release in 2012. Spurning traditional game design, "Dear Esther" features virtually no puzzles or tasks. The player's only objective is to explore an unnamed island in the Hebrides, listening to a troubled man read a series of letters to his deceased wife. Details of her mysterious death are revealed as the player moves throughout the island.  - A video game is an electronic game that involves human or animal interaction with a user interface to generate visual feedback on a video device such as a TV screen or computer monitor. The word "video" in "video game" traditionally referred to a raster display device, but as of the 2000s, it implies any type of display device that can produce two- or three-dimensional images. Some theorists categorize video games as an art form, but this designation is controversial.  - Korsakovia is a single - player mod for Valve Corporation 's video game Half - Life 2 . It was developed by The Chinese Room , known for Dear Esther , and Amnesia : A Machine for Pigs . Korsakovia was released for free download on September 20 , 2009 .  - Left 4 Dead is a cooperative first-person shooter video game with survival horror elements, developed by Valve South and published by Valve Corporation. The game uses Valve's proprietary Source engine, and is available for Microsoft Windows, Xbox 360 and OS X. Development on the game was completed on November 13, 2008, and two versions were released digitally: A downloadable digital version, released on November 17, 2008, and a digital retail disc version, with a release date determined by region. The digital retail disc version was released in North America and Australia on November 18, 2008; and in Europe on November 21, 2008.    After reading the paragraphs above, we are interested in knowing the entity with which 'korsakovia' exhibits the relationship of 'software engine'. Find the answer from the choices below.  Choices: - computer  - source
A:
source