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: Continuous ambulatory ECG monitoring during fluorouracil therapy : a prospective study .
### Output Text:
Continuous ambulatory ECG monitoring during <span class="chemical">fluorouracil</span> therapy : a prospective study . 