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: MATERIALS AND METHODS : Three groups , comprised of 21 cocaine naive participants ( CN ) , 30 occasional cocaine ( OC ) , and 48 regular recreational cocaine ( RC ) users , were compared .
### Output Text:
MATERIALS AND METHODS : Three groups , comprised of 21 <span class="chemical">cocaine</span> naive participants ( CN ) , 30 occasional <span class="chemical">cocaine</span> ( OC ) , and 48 regular recreational <span class="chemical">cocaine</span> ( RC ) users , were compared . 