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 L - W rats , TP acted as a tumor enhancement agent , with primary emphasis on the development of prostate cancer .
### Output Text:
In L - W rats , <span class="chemical">TP</span> acted as a tumor enhancement agent , with primary emphasis on the development of prostate cancer . 