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: Livers were harvested on 2 , 5 , and 10 days during PB administration .
### Output Text:
Livers were harvested on 2 , 5 , and 10 days during <span class="chemical">PB</span> administration . 