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: Subjects with SNHL were submitted to DFO reduction or temporary withdrawal .
### Output Text:
Subjects with SNHL were submitted to <span class="chemical">DFO</span> reduction or temporary withdrawal . 