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: These observations led us to hypothesize that a high intake of vitamin C in diabetic persons might promote atherosclerosis .
### Output Text:
These observations led us to hypothesize that a high intake of <span class="chemical">vitamin C</span> in diabetic persons might promote atherosclerosis . 