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 overall response rate was 38 % , the median time to response was 10 weeks , the median duration of response was 26 weeks , and the median survival was 37 weeks .
### Output Text:
The overall response rate was 38 % , the median time to response was 10 weeks , the median duration of response was 26 weeks , and the median survival was 37 weeks . 