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: Thromboembolic and other complications of oral contraceptive therapy in relationship to pretreatment levels of blood coagulation factors : summary report of a ten - year study .
### Output Text:
Thromboembolic and other complications of <span class="chemical">oral contraceptive</span> therapy in relationship to pretreatment levels of blood coagulation factors : summary report of a ten - year study . 