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 dose of aspirin used in the RCTs varied , which prevented the estimation of the most appropriate dose for primary prevention .
### Output Text:
The dose of <span class="chemical">aspirin</span> used in the RCTs varied , which prevented the estimation of the most appropriate dose for primary prevention . 