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: Logistic regression showed no significant difference in the risk of VTE between users of third - generation and second - generation OCs .
### Output Text:
Logistic regression showed no significant difference in the risk of VTE between users of third - generation and second - generation <span class="chemical">OCs</span> . 