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: Comparison of valsartan / hydrochlorothiazide combination therapy at doses up to 320 / 25 mg versus monotherapy : a double - blind , placebo - controlled study followed by long - term combination therapy in hypertensive adults .
### Output Text:
Comparison of <span class="chemical">valsartan</span> / <span class="chemical">hydrochlorothiazide</span> combination therapy at doses up to 320 / 25 mg versus monotherapy : a double - blind , placebo - controlled study followed by long - term combination therapy in hypertensive adults . 