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: The extent of memory improvement evoked by DCE was 23 % at the dose of 200 mg / kg and 35 % at the dose of 400 mg / kg in young mice using elevated plus maze .
### Output Text:
The extent of memory improvement evoked by <span class="chemical">DCE</span> was 23 % at the dose of 200 mg / kg and 35 % at the dose of 400 mg / kg in young mice using elevated plus maze . 