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: Because Warfarin treatment had no effect on the elevation in serum calcium produced by vitamin D , the synergy between Warfarin and vitamin D is probably best explained by the hypothesis that Warfarin inhibits the activity of matrix Gla protein as a calcification inhibitor .
### Output Text:
Because <span class="chemical">Warfarin</span> treatment had no effect on the elevation in serum <span class="chemical">calcium</span> produced by <span class="chemical">vitamin D</span> , the synergy between <span class="chemical">Warfarin</span> and <span class="chemical">vitamin D</span> is probably best explained by the hypothesis that <span class="chemical">Warfarin</span> inhibits the activity of matrix Gla protein as a calcification inhibitor . 