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: Useful Field of View ( UFOV - - a test of visual attention ) was also undertaken .
### Output Text:
Useful Field of View ( UFOV - - a test of visual attention ) was also undertaken . 