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: Multicenter , double - blind , multiple - dose , parallel - groups efficacy and safety trial of azelastine , chlorpheniramine , and placebo in the treatment of spring allergic rhinitis .
### Output Text:
Multicenter , double - blind , multiple - dose , parallel - groups efficacy and safety trial of <span class="chemical">azelastine</span> , <span class="chemical">chlorpheniramine</span> , and placebo in the treatment of spring allergic rhinitis . 