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: In the hCRH group a marked parallel shift of the CO2 - response curve to the left was observed after hCRH ( P < 0 . 01 ) .
### Output Text:
In the hCRH group a marked parallel shift of the <span class="chemical">CO2</span> - response curve to the left was observed after hCRH ( P < 0 . 01 ) . 