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: We performed a single - blind , placebo - controlled study to assess its effectiveness and safety in Chinese hepatitis B surface antigen ( HBsAg ) carriers .
### Output Text:
We performed a single - blind , placebo - controlled study to assess its effectiveness and safety in Chinese <span class="chemical">hepatitis B surface antigen</span> ( <span class="chemical">HBsAg</span> ) carriers . 