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: As the first dihydropyridine available for use in the United States , nifedipine controls angina and hypertension with minimal depression of cardiac function .
### Output Text:
As the first <span class="chemical">dihydropyridine</span> available for use in the United States , <span class="chemical">nifedipine</span> controls angina and hypertension with minimal depression of cardiac function . 