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: The CO2 - response curves in the control group were nearly identical .
### Output Text:
The <span class="chemical">CO2</span> - response curves in the control group were nearly identical . 