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: Population - based study of risk of venous thromboembolism associated with various oral contraceptives .
### Output Text:
Population - based study of risk of venous thromboembolism associated with various <span class="chemical">oral contraceptives</span> . 