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: The crude rate of VTE per 10 , 000 woman - years was 4 . 10 in current users of any OC , 3 . 10 in users of second - generation OCs , and 4 . 96 in users of third - generation preparations .
### Output Text:
The crude rate of VTE per 10 , 000 woman - years was 4 . 10 in current users of any <span class="chemical">OC</span> , 3 . 10 in users of second - generation <span class="chemical">OCs</span> , and 4 . 96 in users of third - generation preparations . 