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: Group 6 received cocaine and diazepam followed by 10 mg / kg flumazenil .
### Output Text:
Group 6 received <span class="chemical">cocaine</span> and <span class="chemical">diazepam</span> followed by 10 mg / kg <span class="chemical">flumazenil</span> . 