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: Aspirin for the primary prevention of cardiovascular events : an update of the evidence for the U . S .
### Output Text:
<span class="chemical">Aspirin</span> for the primary prevention of cardiovascular events : an update of the evidence for the U . S . 