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: After reduction of ribavirin , 27 of 37 patients could continue combination therapy for a total of 24 weeks ( group A ) .
### Output Text:
After reduction of <span class="chemical">ribavirin</span> , 27 of 37 patients could continue combination therapy for a total of 24 weeks ( group A ) . 