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: Of patients expressing a preference ( 89 . 1 % ) , more than twice as many preferred rizatriptan to ergotamine / caffeine ( 69 . 9 vs . 30 . 1 % , p < or = 0 . 001 ) .
### Output Text:
Of patients expressing a preference ( 89 . 1 % ) , more than twice as many preferred <span class="chemical">rizatriptan</span> to <span class="chemical">ergotamine</span> / <span class="chemical">caffeine</span> ( 69 . 9 vs . 30 . 1 % , p < or = 0 . 001 ) . 