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 : Twenty cocaine - dependent participants were randomly assigned to receive modafinil , 400 mg ( N = 10 ) , or placebo ( N = 10 ) every morning at 7 : 30 a . m . for 16 days in an inpatient , double - blind randomized trial .
### Output Text:
METHOD : Twenty <span class="chemical">cocaine</span> - dependent participants were randomly assigned to receive <span class="chemical">modafinil</span> , 400 mg ( N = 10 ) , or placebo ( N = 10 ) every morning at 7 : 30 a . m . for 16 days in an inpatient , double - blind randomized trial . 