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: 31 of the 38 cases ( 81 . 5 % ) demonstrated objective responses lasting for 2 months or more .
### Output Text:
31 of the 38 cases ( 81 . 5 % ) demonstrated objective responses lasting for 2 months or more . 