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 : An association between the use of oral contraceptives and the risk of myocardial infarction has been found in some , but not all , studies .
### Output Text:
BACKGROUND : An association between the use of <span class="chemical">oral contraceptives</span> and the risk of myocardial infarction has been found in some , but not all , studies . 