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: Owing to a high incidence of thrombophlebitis , the study was terminated prematurely and only the data of the two parallel treatment groups ( 15 patients in each group ) were analysed .
### Output Text:
Owing to a high incidence of thrombophlebitis , the study was terminated prematurely and only the data of the two parallel treatment groups ( 15 patients in each group ) were analysed . 