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The existence of the Colossus machines was kept secret until the mid-1970 s ; the machines and the plans for building them had previously been destroyed in the 1960 s as part of the effort to maintain the secrecy of the project .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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