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: Further studies are needed to determine the most appropriate nucleoside or nucleotide analogue for antiviral prophylaxis during CT and the optimal duration of administration after completion of CT .
### Output Text:
Further studies are needed to determine the most appropriate <span class="chemical">nucleoside</span> or <span class="chemical">nucleotide</span> analogue for antiviral prophylaxis during CT and the optimal duration of administration after completion of CT . 