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: Measures of pupillary oscillation were able to significantly distinguish a group of abstinent crack cocaine abusers endorsing past CIP ( n = 32 ) from another group of crack addicts who denied past CIP ( n = 29 ) .
### Output Text:
Measures of pupillary oscillation were able to significantly distinguish a group of abstinent <span class="chemical">crack cocaine</span> abusers endorsing past CIP ( n = 32 ) from another group of <span class="chemical">crack</span> addicts who denied past CIP ( n = 29 ) . 