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: During the study , vitamin B12 and folate levels were significantly higher in group II patients ; however , no differences in hemoglobin , hematocrit , mean corpuscular volume , and white - cell , neutrophil and platelet counts were observed between groups at 3 , 6 , 9 and 12 months .
### Output Text:
During the study , <span class="chemical">vitamin B12</span> and <span class="chemical">folate</span> levels were significantly higher in group II patients ; however , no differences in hemoglobin , hematocrit , mean corpuscular volume , and white - cell , neutrophil and platelet counts were observed between groups at 3 , 6 , 9 and 12 months . 