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: To determine whether the risk of HIV seroconversion among daily smokers of crack cocaine changed over time , we used Cox proportional hazards regression and divided the study into 3 periods : May 1 , 1996 - Nov .
### Output Text:
To determine whether the risk of HIV seroconversion among daily smokers of <span class="chemical">crack cocaine</span> changed over time , we used Cox proportional hazards regression and divided the study into 3 periods : May 1 , 1996 - Nov . 