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: Acute antinociceptive effects of the combination were partially prevented by 3 . 2 mg / kg naloxone s . c .
### Output Text:
Acute antinociceptive effects of the combination were partially prevented by 3 . 2 mg / kg <span class="chemical">naloxone</span> s . c . 