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: PURPOSE : To determine the benefits and harms of taking aspirin for the primary prevention of myocardial infarctions , strokes , and death .
### Output Text:
PURPOSE : To determine the benefits and harms of taking <span class="chemical">aspirin</span> for the primary prevention of myocardial infarctions , strokes , and death . 