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: BMT was used to treat severe aplastic anemia which was caused by gold in one case and D - penicillamine in the other .
### Output Text:
BMT was used to treat severe aplastic anemia which was caused by <span class="chemical">gold</span> in one case and <span class="chemical">D - penicillamine</span> in the other . 