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: These findings indicate that RA acted as a co - carcinogen in the BHA forestomach carcinogenesis of the rat .
### Output Text:
These findings indicate that <span class="chemical">RA</span> acted as a co - carcinogen in the <span class="chemical">BHA</span> forestomach carcinogenesis of the rat . 