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: Longitudinal assessment of air conduction audiograms in a phase III clinical trial of difluoromethylornithine and sulindac for prevention of sporadic colorectal adenomas .
### Output Text:
Longitudinal assessment of air conduction audiograms in a phase III clinical trial of <span class="chemical">difluoromethylornithine</span> and <span class="chemical">sulindac</span> for prevention of sporadic colorectal adenomas . 