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: It was concluded that labetalol offers advantages over nitroprusside .
### Output Text:
It was concluded that <span class="chemical">labetalol</span> offers advantages over <span class="chemical">nitroprusside</span> . 