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: LIMITATIONS : New evidence on aspirin for the primary prevention of CVD is limited .
### Output Text:
LIMITATIONS : New evidence on <span class="chemical">aspirin</span> for the primary prevention of CVD is limited . 