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: A rat model was developed to examine the effects of chronic CBZ treatment on folate concentrations in the rat .
### Output Text:
A rat model was developed to examine the effects of chronic <span class="chemical">CBZ</span> treatment on <span class="chemical">folate</span> concentrations in the rat . 