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 rates of onset of the action of cimetidine and ranitidine were the same .
### Output Text:
The rates of onset of the action of <span class="chemical">cimetidine</span> and <span class="chemical">ranitidine</span> were the same . 