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: Experimental design consisted of four groups : control ( vehicle alone ) , GSPE alone , drug alone and GSPE + drug .
### Output Text:
Experimental design consisted of four groups : control ( vehicle alone ) , <span class="chemical">GSPE</span> alone , drug alone and <span class="chemical">GSPE</span> + drug . 