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: Nine patients with severe pain in their feet , which was registered after transplantation , were investigated .
### Output Text:
Nine patients with severe pain in their feet , which was registered after transplantation , were investigated . 