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: In June , 1997 , the trialists and the data - monitoring committee decided to end recruitment primarily because of the number of women dropping out of the study .
### Output Text:
In June , 1997 , the trialists and the data - monitoring committee decided to end recruitment primarily because of the number of women dropping out of the study . 