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: All rats were terminated either 24 h or 3 weeks after the DFP injection .
### Output Text:
All rats were terminated either 24 h or 3 weeks after the <span class="chemical">DFP</span> injection . 