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: In normal rats , the whole brain sections exhibited complete staining with TTC .
### Output Text:
In normal rats , the whole brain sections exhibited complete staining with <span class="chemical">TTC</span> . 