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 electrocardiographic ( ECG ) findings for patients in group B were compared with those of patients in groups C1 and C2 .
### Output Text:
The electrocardiographic ( ECG ) findings for patients in group B were compared with those of patients in groups C1 and C2 . 