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: Study of the role of vitamin B12 and folinic acid supplementation in preventing hematologic toxicity of zidovudine .
### Output Text:
Study of the role of <span class="chemical">vitamin B12</span> and <span class="chemical">folinic acid</span> supplementation in preventing hematologic toxicity of <span class="chemical">zidovudine</span> . 