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: In the nine responders , median duration of chemotherapy response from the time of operation was 25 weeks ( range 12 to more than 91 weeks ) .
### Output Text:
In the nine responders , median duration of chemotherapy response from the time of operation was 25 weeks ( range 12 to more than 91 weeks ) . 