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 aim of our study was to re - examine the association between risk of VTE and OC use with a different study design and analysis to avoid some of the bias and confounding of the earlier studies .
### Output Text:
The aim of our study was to re - examine the association between risk of VTE and <span class="chemical">OC</span> use with a different study design and analysis to avoid some of the bias and confounding of the earlier studies . 