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: Reversal of central benzodiazepine effects by flumazenil after intravenous conscious sedation with diazepam and opioids : report of a double - blind multicenter study .
### Output Text:
Reversal of central <span class="chemical">benzodiazepine</span> effects by <span class="chemical">flumazenil</span> after intravenous conscious sedation with <span class="chemical">diazepam</span> and opioids : report of a double - blind multicenter study . 