In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Such factors as sex ( female ) , age ( > or = 60 years old ) , and the ribavirin dose by body weight ( 12 mg / kg or more ) were significant by univariate analysis .
### Output Text:
Such factors as sex ( female ) , age ( > or = 60 years old ) , and the <span class="chemical">ribavirin</span> dose by body weight ( 12 mg / kg or more ) were significant by univariate analysis . 