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 risk of myocardial infarction was similar among women who used oral contraceptives whether or not they had a prothrombotic mutation .
### Output Text:
The risk of myocardial infarction was similar among women who used <span class="chemical">oral contraceptives</span> whether or not they had a prothrombotic mutation . 