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 most dramatic hemodynamic changes occurred after each cocaine bolus in the C + E and C only groups ; however , persistent hemodynamic changes occurred in the C + E group .
### Output Text:
The most dramatic hemodynamic changes occurred after each <span class="chemical">cocaine</span> bolus in the C + E and C only groups ; however , persistent hemodynamic changes occurred in the C + E group . 