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: We conclude that the risk of the serious diseases studied is small for the three agents , and compares reasonably with the risk for many other antibiotics .
### Output Text:
We conclude that the risk of the serious diseases studied is small for the three agents , and compares reasonably with the risk for many other antibiotics . 