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: Previous studies have demonstrated an increased risk of venous thromboembolism ( VTE ) associated with CPA / EE compared with conventional combined oral contraceptives ( COCs ) .
### Output Text:
Previous studies have demonstrated an increased risk of venous thromboembolism ( VTE ) associated with <span class="chemical">CPA</span> / <span class="chemical">EE</span> compared with conventional combined <span class="chemical">oral contraceptives</span> ( COCs ) . 