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: On postnatal day ( PND ) 35 , male mice were exposed to 50mg BPA / kg diet per day for a period of 35 days .
### Output Text:
On postnatal day ( PND ) 35 , male mice were exposed to 50mg <span class="chemical">BPA</span> / kg diet per day for a period of 35 days . 