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: BACKGROUND : Vitamin C acts as a potent antioxidant ; however , it can also be a prooxidant and glycate protein under certain circumstances in vitro .
### Output Text:
BACKGROUND : <span class="chemical">Vitamin C</span> acts as a potent antioxidant ; however , it can also be a prooxidant and glycate protein under certain circumstances in vitro . 