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: Clinical applications of calcium channel blockers parallel their tissue selectivity .
### Output Text:
Clinical applications of <span class="chemical">calcium channel blockers</span> parallel their tissue selectivity . 