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: OBJECTIVES : Recently published studies have demonstrated increased efficacy and cost - effectiveness of combination therapy with interferon and alpha - 2b / ribavirin compared with interferon - alpha monotherapy in the treatment of chronic hepatitis C ( CHC ) .
### Output Text:
OBJECTIVES : Recently published studies have demonstrated increased efficacy and cost - effectiveness of combination therapy with interferon and alpha - 2b / <span class="chemical">ribavirin</span> compared with <span class="chemical">interferon - alpha</span> monotherapy in the treatment of chronic hepatitis C ( CHC ) . 