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### Input Text: As a result of these and other studies , phenacetin has now been withdrawn from the market in most countries .
### Output Text:
As a result of these and other studies , <span class="chemical">phenacetin</span> has now been withdrawn from the market in most countries . 