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: OBJECTIVE : To report a case of central retinal vein occlusion associated with clomiphene citrate ( CC ) .
### Output Text:
OBJECTIVE : To report a case of central retinal vein occlusion associated with <span class="chemical">clomiphene citrate</span> ( <span class="chemical">CC</span> ) . 