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 , acetaminophen has been used both as a single agent and in combination with other analgesics , whereas phenacetin was available only in combinations .
### Output Text:
Thus , <span class="chemical">acetaminophen</span> has been used both as a single agent and in combination with other analgesics , whereas <span class="chemical">phenacetin</span> was available only in combinations . 