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 aim of the present study was to determine the efficacy of glyceryl trinitrate ( GTN ) , an NO donor , in the resolution of primary dysmenorrhea in comparison with diclofenac ( DCF ) .
### Output Text:
The aim of the present study was to determine the efficacy of <span class="chemical">glyceryl trinitrate</span> ( <span class="chemical">GTN</span> ) , an <span class="chemical">NO</span> donor , in the resolution of primary dysmenorrhea in comparison with <span class="chemical">diclofenac</span> ( <span class="chemical">DCF</span> ) . 