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 increased odds ratio associated with products containing 20 micrograms ethinyloestradiol and desogestrel compared with the 30 micrograms product is biologically implausible , and is likely to be the result of preferential prescribing and , thus , confounding .
### Output Text:
The increased odds ratio associated with products containing 20 micrograms <span class="chemical">ethinyloestradiol</span> and <span class="chemical">desogestrel</span> compared with the 30 micrograms product is biologically implausible , and is likely to be the result of preferential prescribing and , thus , confounding . 