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: BACKGROUND AND AIM : Interferon and ribavirin combination therapy for chronic hepatitis C produces hemolytic anemia .
### Output Text:
BACKGROUND AND AIM : <span class="chemical">Interferon</span> and <span class="chemical">ribavirin</span> combination therapy for chronic hepatitis C produces hemolytic anemia . 