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 of the extreme toxicity of vitamin D3 in pregnant Jersey cows and the low margin of safety between doses of vitamin D3 that prevent milk fever and doses that induce milk fever , we concluded that vitamin D3 cannot be used practically to prevent milk fever when injected several weeks prepartum .
### Output Text:
Because of the extreme toxicity of <span class="chemical">vitamin D3</span> in pregnant Jersey cows and the low margin of safety between doses of <span class="chemical">vitamin D3</span> that prevent milk fever and doses that induce milk fever , we concluded that <span class="chemical">vitamin D3</span> cannot be used practically to prevent milk fever when injected several weeks prepartum . 