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: The subjects were randomly allocated to receive 0 . 5 microg / kg / min GTN or placebo over 20 min on two headache - free days .
### Output Text:
The subjects were randomly allocated to receive 0 . 5 microg / kg / min <span class="chemical">GTN</span> or placebo over 20 min on two headache - free days . 