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: OBJECTIVE : To study the 2 drugs most clearly implicated ( clozapine and olanzapine ) and risperidone using a frequently sampled intravenous glucose tolerance test .
### Output Text:
OBJECTIVE : To study the 2 drugs most clearly implicated ( <span class="chemical">clozapine</span> and <span class="chemical">olanzapine</span> ) and <span class="chemical">risperidone</span> using a frequently sampled intravenous <span class="chemical">glucose</span> tolerance test . 