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: Given the increasing use of crack cocaine , we sought to examine whether use of this illicit drug has become a risk factor for HIV infection .
### Output Text:
Given the increasing use of <span class="chemical">crack cocaine</span> , we sought to examine whether use of this illicit drug has become a risk factor for HIV infection . 