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: METHODS : Fourteen patients chronically infected with hepatitis C virus were treated by pegylated interferon alpha 2a and ribavirin .
### Output Text:
METHODS : Fourteen patients chronically infected with hepatitis C virus were treated by <span class="chemical">pegylated interferon alpha 2a</span> and <span class="chemical">ribavirin</span> . 