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: ( 6 subjects ) whereas 11 received theophylline at 300 mg b . i . d .
### Output Text:
( 6 subjects ) whereas 11 received <span class="chemical">theophylline</span> at 300 mg b . i . d . 