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: Rizatriptan is a selective 5 - HT ( 1B / 1D ) receptor agonist with rapid oral absorption and early onset of action in the acute treatment of migraine .
### Output Text:
<span class="chemical">Rizatriptan</span> is a selective <span class="chemical">5 - HT</span> ( 1B / 1D ) receptor agonist with rapid oral absorption and early onset of action in the acute treatment of migraine . 