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: RESULTS : During the time frame of the study , 330 women were found to have had VTE while on COCs .
### Output Text:
RESULTS : During the time frame of the study , 330 women were found to have had VTE while on <span class="chemical">COCs</span> . 