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 difference between the bolus and infusion groups in the percentage of participants who saw a 50 % reduction in their headache intensity within 30 minutes was 11 . 8 % ( 95 % CI - 9 . 6 % to 33 . 3 % ) .
### Output Text:
The difference between the bolus and infusion groups in the percentage of participants who saw a 50 % reduction in their headache intensity within 30 minutes was 11 . 8 % ( 95 % CI - 9 . 6 % to 33 . 3 % ) . 