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 ratio of cimetidine clearance to creatinine clearance ( Rc ) averaged 4 . 8 + / - 2 . 0 , indicating net tubular secretion for cimetidine .
### Output Text:
The ratio of <span class="chemical">cimetidine</span> clearance to <span class="chemical">creatinine</span> clearance ( Rc ) averaged 4 . 8 + / - 2 . 0 , indicating net tubular secretion for <span class="chemical">cimetidine</span> . 