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: There was a close parallel between the effect of vitamin D dose on artery calcification and the effect of vitamin D dose on the elevation of serum calcium , which suggests that vitamin D may induce artery calcification through its effect on serum calcium .
### Output Text:
There was a close parallel between the effect of <span class="chemical">vitamin D</span> dose on artery calcification and the effect of <span class="chemical">vitamin D</span> dose on the elevation of serum <span class="chemical">calcium</span> , which suggests that <span class="chemical">vitamin D</span> may induce artery calcification through its effect on serum <span class="chemical">calcium</span> . 