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: Aggregation was lower ( P < . 05 ) in the ketoprofen group than in the acetaminophen group just before surgery and on the third postoperative day .
### Output Text:
Aggregation was lower ( P < . 05 ) in the <span class="chemical">ketoprofen</span> group than in the <span class="chemical">acetaminophen</span> group just before surgery and on the third postoperative day . 