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: UNASSIGNED : Metolachlor , a widely used herbicide , is classified as a Group C carcinogen by the U . S .
### Output Text:
UNASSIGNED : <span class="chemical">Metolachlor</span> , a widely used herbicide , is classified as a Group C carcinogen by the U . S . 