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: The adjusted rate ratio of VTE for repeat users of third generation OC was 0 . 6 ( 95 % CI : 0 . 3 - 1 . 2 ) relative to repeat users of second generation pills , whereas it was 1 . 3 ( 95 % CI : 0 . 7 - 2 . 4 ) for switchers from second to third generation pills relative to switchers from third to second generation pills .
### Output Text:
The adjusted rate ratio of VTE for repeat users of third generation <span class="chemical">OC</span> was 0 . 6 ( 95 % CI : 0 . 3 - 1 . 2 ) relative to repeat users of second generation pills , whereas it was 1 . 3 ( 95 % CI : 0 . 7 - 2 . 4 ) for switchers from second to third generation pills relative to switchers from third to second generation pills . 