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 observed positive impact of nifedipine on reducing the nephrotoxicity associated with tacrolimus in liver transplant recipients should be an important factor in selecting an agent to treat hypertension in this population .
### Output Text:
The observed positive impact of <span class="chemical">nifedipine</span> on reducing the nephrotoxicity associated with <span class="chemical">tacrolimus</span> in liver transplant recipients should be an important factor in selecting an agent to treat hypertension in this population . 