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 drug effect studied was the antagonism by metoprolol of terbutaline - induced hypokalemia .
### Output Text:
The drug effect studied was the antagonism by <span class="chemical">metoprolol</span> of <span class="chemical">terbutaline</span> - induced hypokalemia . 