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: During withdrawal ( days 8 and 9 ) , these mice were administered Re , Rg1 , or Rb1 intragastrically two times per day .
### Output Text:
During withdrawal ( days 8 and 9 ) , these mice were administered <span class="chemical">Re</span> , <span class="chemical">Rg1</span> , or <span class="chemical">Rb1</span> intragastrically two times per day . 