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: During treatment of chronic hepatitis C patients with interferon and ribavirin , a lot of side effects are described .
### Output Text:
During treatment of chronic hepatitis C patients with interferon and <span class="chemical">ribavirin</span> , a lot of side effects are described . 