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: VNB was administered weekly as a 25 - mg / m2 short intravenous ( i . v . ) infusion .
### Output Text:
<span class="chemical">VNB</span> was administered weekly as a 25 - mg / m2 short intravenous ( i . v . ) infusion . 