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: RESULTS : In group 1 , animals received cocaine followed by vehicle .
### Output Text:
RESULTS : In group 1 , animals received <span class="chemical">cocaine</span> followed by vehicle . 