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: C57BL / 6J , chosen as a " prototypic " mouse strain , was used to determine behavioral responses to a broad range ( 5 - 500 mg / kg ) of caffeine doses .
### Output Text:
C57BL / 6J , chosen as a " prototypic " mouse strain , was used to determine behavioral responses to a broad range ( 5 - 500 mg / kg ) of <span class="chemical">caffeine</span> doses . 