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: BACKGROUND : More than 3 decades after Jones and Smith ( 1973 ) reported on the devastation caused by alcohol exposure on fetal development , the rates of heavy drinking during pregnancy remain relatively unchanged .
### Output Text:
BACKGROUND : More than 3 decades after Jones and Smith ( 1973 ) reported on the devastation caused by <span class="chemical">alcohol</span> exposure on fetal development , the rates of heavy drinking during pregnancy remain relatively unchanged . 