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: Locomotor activity was measured again for 30 min at the beginning of days 1 and 21 of sugar access .
### Output Text:
Locomotor activity was measured again for 30 min at the beginning of days 1 and 21 of sugar access . 