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: Two patients attained a complete response ( 4 % ) and 11 patients ( 22 % ) achieved a partial response .
### Output Text:
Two patients attained a complete response ( 4 % ) and 11 patients ( 22 % ) achieved a partial response . 