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: Male SD rats ( n = 30 ) were treated with Ato ( 50 mg / kg per day in drinking water ) or tap water for 15 days .
### Output Text:
Male SD rats ( n = 30 ) were treated with <span class="chemical">Ato</span> ( 50 mg / kg per day in drinking water ) or tap water for 15 days . 