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: All 20 patients responded to this regimen , 16 / 20 ( 80 % ) achieved a complete remission , and 20 % obtained a partial remission .
### Output Text:
All 20 patients responded to this regimen , 16 / 20 ( 80 % ) achieved a complete remission , and 20 % obtained a partial remission . 