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: Use of BZDs / RDs tended to be associated with a reduced ability to walk and shorter night - time sleep during the week prior to admission .
### Output Text:
Use of <span class="chemical">BZDs</span> / RDs tended to be associated with a reduced ability to walk and shorter night - time sleep during the week prior to admission . 