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: These analyses suggest that the higher risk observed for the newer OC in other studies may be the result of inadequate comparisons of pill users with different patterns of pill use .
### Output Text:
These analyses suggest that the higher risk observed for the newer <span class="chemical">OC</span> in other studies may be the result of inadequate comparisons of pill users with different patterns of pill use . 