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: Administration of DDAVP which has antidiuretic action but minimal vasopressor effect failed to increase blood pressure to the levels observed after administration of AVP .
### Output Text:
Administration of <span class="chemical">DDAVP</span> which has antidiuretic action but minimal vasopressor effect failed to increase blood pressure to the levels observed after administration of AVP . 