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: Thus the mechanism by which acetaminophen antagonizes the actions of caffeine in the CNS remains unknown .
### Output Text:
Thus the mechanism by which <span class="chemical">acetaminophen</span> antagonizes the actions of <span class="chemical">caffeine</span> in the CNS remains unknown . 