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: MAIN OUTCOME MEASURE ( S ) : Central retinal vein occlusion after ovulation induction with CC .
### Output Text:
MAIN OUTCOME MEASURE ( S ) : Central retinal vein occlusion after ovulation induction with <span class="chemical">CC</span> . 