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 the 83 cases of VTE associated with use of combined OCs , 43 were recorded as deep - vein thrombosis , 35 as pulmonary thrombosis , and five as venous thrombosis not otherwise specified .
### Output Text:
Of the 83 cases of VTE associated with use of combined <span class="chemical">OCs</span> , 43 were recorded as deep - vein thrombosis , 35 as pulmonary thrombosis , and five as venous thrombosis not otherwise specified . 