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: OBJECTIVE : To describe associations between the use of benzodiazepines or related drugs ( BZDs / RDs ) and health , functional abilities and cognitive function in the elderly .
### Output Text:
OBJECTIVE : To describe associations between the use of <span class="chemical">benzodiazepines</span> or related drugs ( <span class="chemical">BZDs</span> / RDs ) and health , functional abilities and cognitive function in the elderly . 