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: Postmarketing study of timolol - hydrochlorothiazide antihypertensive therapy .
### Output Text:
Postmarketing study of <span class="chemical">timolol</span> - <span class="chemical">hydrochlorothiazide</span> antihypertensive therapy . 