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: Two hundred four people with epilepsy were grouped on the basis of antiepileptic drug therapy ( current , previous , or no exposure to VGB ) .
### Output Text:
Two hundred four people with epilepsy were grouped on the basis of antiepileptic drug therapy ( current , previous , or no exposure to <span class="chemical">VGB</span> ) . 