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: In contrast , the fear - potentiated startle ( FPS ) is suggested to model conditioned fear .
### Output Text:
In contrast , the fear - potentiated startle ( FPS ) is suggested to model conditioned fear . 