5824
Phil Williams , in an overview article , states that " violent non-state actors ( VNSAs ) have become a pervasive challenge to nation-states " in the 21st century " .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 None