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Across species the selenium : mercury molar ratio decreased with increasing mean mercury levels , but selenium variation also contributed to the ratio .
 Chemical: selenium; Chemical: mercury; Chemical: mercury; Chemical: selenium
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: selenium; Chemical: mercury; Chemical: mercury; Chemical: selenium