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: Responder rates at 8 weeks were 66 . 7 % for valsartan and 60 . 2 % for amlodipine ( p = 0 . 39 ) .
### Output Text:
Responder rates at 8 weeks were 66 . 7 % for <span class="chemical">valsartan</span> and 60 . 2 % for <span class="chemical">amlodipine</span> ( p = 0 . 39 ) . 