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 safety of the zinc acetate allowed us to avoid other potentially toxic chelating drugs ; this observation is in line with the growing evidence on the efficacy of the drug in the treatment of Wilson ' s disease .
### Output Text:
The safety of the <span class="chemical">zinc acetate</span> allowed us to avoid other potentially toxic chelating drugs ; this observation is in line with the growing evidence on the efficacy of the drug in the treatment of Wilson ' s disease . 