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: This was an exploratory study to investigate lamivudine - resistant hepatitis B virus ( HBV ) strains in selected lamivudine - na ve HBV carriers with and without human immunodeficiency virus ( HIV ) co - infection in South African patients .
### Output Text:
This was an exploratory study to investigate <span class="chemical">lamivudine</span> - resistant hepatitis B virus ( HBV ) strains in selected <span class="chemical">lamivudine</span> - <span class="chemical">na</span> ve HBV carriers with and without human immunodeficiency virus ( HIV ) co - infection in South African patients . 