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: Animals used at M0 ( n = 8 ) were also used at moment - 24 h of acute study .
### Output Text:
Animals used at M0 ( n = 8 ) were also used at moment - 24 h of acute study . 