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: When dietary and supplemental vitamin C were analyzed separately , only supplemental vitamin C showed a positive association with mortality endpoints .
### Output Text:
When dietary and supplemental <span class="chemical">vitamin C</span> were analyzed separately , only supplemental <span class="chemical">vitamin C</span> showed a positive association with mortality endpoints . 