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: A phase I / II study of paclitaxel plus cisplatin as first - line therapy for head and neck cancers : preliminary results .
### Output Text:
A phase I / II study of <span class="chemical">paclitaxel</span> plus <span class="chemical">cisplatin</span> as first - line therapy for head and neck cancers : preliminary results . 