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 most significant predictors were duration of levodopa therapy , total daily levodopa dose , and onset age .
### Output Text:
The most significant predictors were duration of <span class="chemical">levodopa</span> therapy , total daily <span class="chemical">levodopa</span> dose , and onset age . 