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: The second set of experiments examined the possible synergy between vitamin D and Warfarin in artery calcification .
### Output Text:
The second set of experiments examined the possible synergy between <span class="chemical">vitamin D</span> and <span class="chemical">Warfarin</span> in artery calcification . 