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: Summed stress and summed difference scores were not significantly reduced , although they were 18 % and 30 % lower , respectively , than control .
### Output Text:
Summed stress and summed difference scores were not significantly reduced , although they were 18 % and 30 % lower , respectively , than control . 