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: Of these women , 67 were on levonorgestrel - containing COCs .
### Output Text:
Of these women , 67 were on <span class="chemical">levonorgestrel</span> - containing <span class="chemical">COCs</span> . 