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: Ninety - nine percent ( sixty - eight ) of the sixty - nine parents present during the reduction were pleased with the sedation and would allow it to be used again in a similar situation .
### Output Text:
Ninety - nine percent ( sixty - eight ) of the sixty - nine parents present during the reduction were pleased with the sedation and would allow it to be used again in a similar situation . 