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: The mean proportion of participants who reported daily smoking of crack cocaine increased from 11 . 6 % in period 1 to 39 . 7 % in period 3 .
### Output Text:
The mean proportion of participants who reported daily smoking of <span class="chemical">crack cocaine</span> increased from 11 . 6 % in period 1 to 39 . 7 % in period 3 . 