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: BACKGROUND : Four studies published since December , 1995 , reported that the incidence of venous thromboembolism ( VTE ) was higher in women who used oral contraceptives ( OCs ) containing the third - generation progestagens gestodene or desogestrel than in users of OCs containing second - generation progestagens .
### Output Text:
BACKGROUND : Four studies published since December , 1995 , reported that the incidence of venous thromboembolism ( VTE ) was higher in women who used <span class="chemical">oral contraceptives</span> ( <span class="chemical">OCs</span> ) containing the third - generation <span class="chemical">progestagens</span> <span class="chemical">gestodene</span> or <span class="chemical">desogestrel</span> than in users of <span class="chemical">OCs</span> containing second - generation <span class="chemical">progestagens</span> . 