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: Patients in group A had significant mortality at 2 - year follow - up ( 28 % ) , in contrast to zero mortality in the other three groups .
### Output Text:
Patients in group A had significant mortality at 2 - year follow - up ( 28 % ) , in contrast to zero mortality in the other three groups . 