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: The clinical utility of dopamine ( DA ) D1 receptor agonists in the treatment of Parkinson ' s disease ( PD ) is still unclear .
### Output Text:
The clinical utility of <span class="chemical">dopamine</span> ( <span class="chemical">DA</span> ) D1 receptor agonists in the treatment of Parkinson ' s disease ( PD ) is still unclear . 