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: Absolute and attributable risk of venous thromboembolism in women on combined cyproterone acetate and ethinylestradiol .
### Output Text:
Absolute and attributable risk of venous thromboembolism in women on combined <span class="chemical">cyproterone acetate</span> and <span class="chemical">ethinylestradiol</span> . 