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: With all second - generation OCs as the reference , the odds ratios for VTE were 3 . 49 ( 1 . 21 - 10 . 12 ) for desogestrel plus 20 g ethinyloestradiol and 1 . 18 ( 0 . 66 - 2 . 17 ) for the other third - generation progestagens .
### Output Text:
With all second - generation <span class="chemical">OCs</span> as the reference , the odds ratios for VTE were 3 . 49 ( 1 . 21 - 10 . 12 ) for <span class="chemical">desogestrel</span> plus 20 g <span class="chemical">ethinyloestradiol</span> and 1 . 18 ( 0 . 66 - 2 . 17 ) for the other third - generation <span class="chemical">progestagens</span> . 