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: METHOD : In a 16 - week multicenter , double - blind trial , 122 children with ADHD were randomly assigned to clonidine ( n = 31 ) , methylphenidate ( n = 29 ) , clonidine and methylphenidate ( n = 32 ) , or placebo ( n = 30 ) .
### Output Text:
METHOD : In a 16 - week multicenter , double - blind trial , 122 children with ADHD were randomly assigned to <span class="chemical">clonidine</span> ( n = 31 ) , <span class="chemical">methylphenidate</span> ( n = 29 ) , <span class="chemical">clonidine</span> and <span class="chemical">methylphenidate</span> ( n = 32 ) , or placebo ( n = 30 ) . 