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: The present study was designed to investigate the hypothesis that calcium channel blockers in general induce cardiovascular malformations indicating a pharmacologic class effect .
### Output Text:
The present study was designed to investigate the hypothesis that <span class="chemical">calcium</span> channel blockers in general induce cardiovascular malformations indicating a pharmacologic class effect . 