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: BACKGROUND : Little is known about the possible role that smoking crack cocaine has on the incidence of HIV infection .
### Output Text:
BACKGROUND : Little is known about the possible role that smoking <span class="chemical">crack cocaine</span> has on the incidence of HIV infection . 