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: INTERPRETATION : Smoking of crack cocaine was found to be an independent risk factor for HIV seroconversion among people who were injection drug users .
### Output Text:
INTERPRETATION : Smoking of <span class="chemical">crack cocaine</span> was found to be an independent risk factor for HIV seroconversion among people who were injection drug users . 