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: Four days after carboplatin treatment , tumors ( DS - sarcoma of the rat ) were implanted s . c . onto the hind food dorsum .
### Output Text:
Four days after <span class="chemical">carboplatin</span> treatment , tumors ( DS - sarcoma of the rat ) were implanted s . c . onto the hind food dorsum . 