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: The present study was aimed at investigating the effects of Daucus carota seeds on cognitive functions , total serum cholesterol levels and brain cholinesterase activity in mice .
### Output Text:
The present study was aimed at investigating the effects of Daucus carota seeds on cognitive functions , total serum <span class="chemical">cholesterol</span> levels and brain cholinesterase activity in mice . 