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: This study shows that busulfan neurotoxicity is dose - dependent in children and efficiently prevented by clonazepam .
### Output Text:
This study shows that <span class="chemical">busulfan</span> neurotoxicity is dose - dependent in children and efficiently prevented by <span class="chemical">clonazepam</span> . 