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: RESULTS : The mean + / - SD duration of treatment with the identified atypical antipsychotic agent was 68 . 3 + / - 28 . 9 months ( clozapine ) , 29 . 5 + / - 17 . 5 months ( olanzapine ) , and 40 . 9 + / - 33 . 7 ( risperidone ) .
### Output Text:
RESULTS : The mean + / - SD duration of treatment with the identified atypical antipsychotic agent was 68 . 3 + / - 28 . 9 months ( <span class="chemical">clozapine</span> ) , 29 . 5 + / - 17 . 5 months ( <span class="chemical">olanzapine</span> ) , and 40 . 9 + / - 33 . 7 ( <span class="chemical">risperidone</span> ) . 