3974
By 1951 , those identified in " Red Channels " were blacklisted across much or all of the movie and broadcast industries unless they cleared their names , the customary requirement being that they testify before the House Un-American Activities Committee .
 House Un-American Activities Committee
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 House Un-American Activities Committee