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: Frequencies of ventricular fibrillation were significantly lower ( p less than 0 . 05 ) after iopentol ( 0 % ) and iohexol ( 3 % ) than after metrizoate ( 22 % ) .
### Output Text:
Frequencies of ventricular fibrillation were significantly lower ( p less than 0 . 05 ) after <span class="chemical">iopentol</span> ( 0 % ) and <span class="chemical">iohexol</span> ( 3 % ) than after <span class="chemical">metrizoate</span> ( 22 % ) . 