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: In the case - control study , we matched cases to controls by exact year of birth , practice , and current use of OCs .
### Output Text:
In the case - control study , we matched cases to controls by exact year of birth , practice , and current use of <span class="chemical">OCs</span> . 