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: The possible beneficial effect of ribavirin during the initial days of AHF is discussed .
### Output Text:
The possible beneficial effect of <span class="chemical">ribavirin</span> during the initial days of AHF is discussed . 