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: STUDY SELECTION : English - language randomized , controlled trials ( RCTs ) ; case - control studies ; meta - analyses ; and systematic reviews of aspirin versus control for the primary prevention of cardiovascular disease ( CVD ) were selected to answer the following questions : Does aspirin decrease coronary heart events , strokes , death from coronary heart events or stroke , or all - cause mortality in adults without known CVD ?
### Output Text:
STUDY SELECTION : English - language randomized , controlled trials ( RCTs ) ; case - control studies ; meta - analyses ; and systematic reviews of <span class="chemical">aspirin</span> versus control for the primary prevention of cardiovascular disease ( CVD ) were selected to answer the following questions : Does <span class="chemical">aspirin</span> decrease coronary heart events , strokes , death from coronary heart events or stroke , or all - cause mortality in adults without known CVD ? 