24489
METHOD : A group of current heavy smokers ( n = 24 ) , ex - smokers who were abstinent for at least 5 years ( n = 20 ) , and never - smokers ( n = 20 ) took part in the experiment .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None