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: A low incidence of cardiovascular malformations was observed after exposure to each of the four calcium channel blockers , but this incidence was statistically significant only for verapamil and nifedipine .
### Output Text:
A low incidence of cardiovascular malformations was observed after exposure to each of the four <span class="chemical">calcium</span> channel blockers , but this incidence was statistically significant only for <span class="chemical">verapamil</span> and <span class="chemical">nifedipine</span> . 