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: We used a multiple logistic regression model that included body - mass index , number of cycles , change in type of OC prescribed within 3 months of the event , previous pregnancy , and concurrent disease .
### Output Text:
We used a multiple logistic regression model that included body - mass index , number of cycles , change in type of <span class="chemical">OC</span> prescribed within 3 months of the event , previous pregnancy , and concurrent disease . 