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 DEN + PB treated rats , the survival was prolonged and the tumor incidence decreased as compared with the results obtained by DEN alone .
### Output Text:
In <span class="chemical">DEN</span> + <span class="chemical">PB</span> treated rats , the survival was prolonged and the tumor incidence decreased as compared with the results obtained by <span class="chemical">DEN</span> alone . 