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: Finally , a pair - fed ( PF ) study , performed in 18 rats ( C , T , and PF ) , showed that muscle atrophy was considerably less pronounced in PF animals than in T - treated animals .
### Output Text:
Finally , a pair - fed ( PF ) study , performed in 18 rats ( C , <span class="chemical">T</span> , and PF ) , showed that muscle atrophy was considerably less pronounced in PF animals than in <span class="chemical">T</span> - treated animals . 