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: Of 18 patients evaluable for response , seven ( 39 % ) achieved a complete response and six ( 33 % ) achieved a partial response .
### Output Text:
Of 18 patients evaluable for response , seven ( 39 % ) achieved a complete response and six ( 33 % ) achieved a partial response . 