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: For each of the three tested calcium channel blockers ( diltiazem , verapamil and bepridil ) 6 groups of mice were treated by two different doses , i . e . 2 and 10 mg / kg / i . p . , or an equal volume of saline for the control group ( n = 20 ) ; 15 minutes later , all the animals were injected with a single 50 mg / kg / i . p . dose of bupivacaine .
### Output Text:
For each of the three tested <span class="chemical">calcium</span> channel blockers ( <span class="chemical">diltiazem</span> , <span class="chemical">verapamil</span> and <span class="chemical">bepridil</span> ) 6 groups of mice were treated by two different doses , i . e . 2 and 10 mg / kg / i . p . , or an equal volume of saline for the control group ( n = 20 ) ; 15 minutes later , all the animals were injected with a single 50 mg / kg / i . p . dose of <span class="chemical">bupivacaine</span> . 