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: Ascending dose tolerance study of intramuscular carbetocin administered after normal vaginal birth .
### Output Text:
Ascending dose tolerance study of intramuscular <span class="chemical">carbetocin</span> administered after normal vaginal birth . 