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: Thus , the relationship between cholinergic activity and responsiveness and alcohol withdrawal was investigated in a genetic animal model of ethanol withdrawal severity .
### Output Text:
Thus , the relationship between cholinergic activity and responsiveness and <span class="chemical">alcohol</span> withdrawal was investigated in a genetic animal model of <span class="chemical">ethanol</span> withdrawal severity . 