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: Participants in the Medical Research Council treatment trial for mild hypertension are randomly allocated to one of four treatment groups : bendrofluazide , propranolol , or a placebo for either of these drugs .
### Output Text:
Participants in the Medical Research Council treatment trial for mild hypertension are randomly allocated to one of four treatment groups : <span class="chemical">bendrofluazide</span> , <span class="chemical">propranolol</span> , or a placebo for either of these drugs . 