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: Seventeen of these had a response or were stable for a median of 20 weeks ( range 6 to more than 66 weeks ) .
### Output Text:
Seventeen of these had a response or were stable for a median of 20 weeks ( range 6 to more than 66 weeks ) . 