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 , caffeine ( 3 . 3 mg / kg ) versus placebo was tested in 48 healthy men ( aged 20 to 35 years ) selected after screening on 2 separate occasions .
### Output Text:
Thus , <span class="chemical">caffeine</span> ( 3 . 3 mg / kg ) versus placebo was tested in 48 healthy men ( aged 20 to 35 years ) selected after screening on 2 separate occasions . 