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: In contrast , D - MED prevented alfentanil - induced muscle rigidity in a dose - dependent fashion .
### Output Text:
In contrast , <span class="chemical">D - MED</span> prevented <span class="chemical">alfentanil</span> - induced muscle rigidity in a dose - dependent fashion . 