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: Two or three BZDs / RDs were concomitantly taken by 26 % of users ( n = 20 ) .
### Output Text:
Two or three <span class="chemical">BZDs</span> / RDs were concomitantly taken by 26 % of users ( n = 20 ) . 