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: CONCLUSION ( S ) : This is the first reported case of central retinal vein occlusion after treatment with CC .
### Output Text:
CONCLUSION ( S ) : This is the first reported case of central retinal vein occlusion after treatment with <span class="chemical">CC</span> . 