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: Additional members of this group of calcium channel blockers have been studied for a variety of indications for which they may offer advantages over current therapy .
### Output Text:
Additional members of this group of <span class="chemical">calcium channel blockers</span> have been studied for a variety of indications for which they may offer advantages over current therapy . 