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 results with respect to the use of third - generation oral contraceptives were inconclusive but suggested that the risk was lower than the risk associated with second - generation oral contraceptives .
### Output Text:
The results with respect to the use of third - generation <span class="chemical">oral contraceptives</span> were inconclusive but suggested that the risk was lower than the risk associated with second - generation <span class="chemical">oral contraceptives</span> . 