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: After adjustment for age , the rate ratio of VTE in users of third - generation relative to second - generation OCs was 1 . 68 ( 95 % CI 1 . 04 - 2 . 75 ) .
### Output Text:
After adjustment for age , the rate ratio of VTE in users of third - generation relative to second - generation <span class="chemical">OCs</span> was 1 . 68 ( 95 % CI 1 . 04 - 2 . 75 ) . 