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: Because salicylates have been reported to augment the stimulatory effects of caffeine on the CNS , attention was focused on the possibility that the presence of acetaminophen ( 52 micrograms / mL ) reduced the CNS toxicity of caffeine .
### Output Text:
Because salicylates have been reported to augment the stimulatory effects of <span class="chemical">caffeine</span> on the CNS , attention was focused on the possibility that the presence of <span class="chemical">acetaminophen</span> ( 52 micrograms / mL ) reduced the CNS toxicity of <span class="chemical">caffeine</span> . 