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: We studied three calcium channel blockers of different structure , nifedipine , diltiazem , and verapamil , along with the new agent .
### Output Text:
We studied three <span class="chemical">calcium</span> channel blockers of different structure , <span class="chemical">nifedipine</span> , <span class="chemical">diltiazem</span> , and <span class="chemical">verapamil</span> , along with the new agent . 