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: Castration surgery was performed in another group of rats , and the passive avoidance task was assessed 4 weeks after the operation .
### Output Text:
Castration surgery was performed in another group of rats , and the passive avoidance task was assessed 4 weeks after the operation . 