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: Furthermore , at 5 , 10 and 20 mg / kg , trazodone blocks 5 - HT 2A and 5 - HT 2C receptors .
### Output Text:
Furthermore , at 5 , 10 and 20 mg / kg , <span class="chemical">trazodone</span> blocks <span class="chemical">5 - HT</span> 2A and <span class="chemical">5 - HT</span> 2C receptors . 