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: RESULTS : The mean + / - SD duration of BZD / RD use was 7 + / - 7 years ( range 1 - 31 ) .
### Output Text:
RESULTS : The mean + / - SD duration of BZD / RD use was 7 + / - 7 years ( range 1 - 31 ) . 