9991
2. Our study involved a group of 60 male and female subjects who were exposed to MTBE and benzene -contaminated water concentrations up to 76 PPB for MTBE and 14 PPB for benzene , for a period of 5 to 8 years.
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 None