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: OBJECTIVE : We report our experience with intravenous ribavirin therapy for severe adenovirus disease in a series of immunocompromised children and review the literature .
### Output Text:
OBJECTIVE : We report our experience with intravenous <span class="chemical">ribavirin</span> therapy for severe adenovirus disease in a series of immunocompromised children and review the literature . 