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: Comparison of the effectiveness of ranitidine and cimetidine in inhibiting acid secretion in patients with gastric hypersecretory states .
### Output Text:
Comparison of the effectiveness of <span class="chemical">ranitidine</span> and <span class="chemical">cimetidine</span> in inhibiting acid secretion in patients with gastric hypersecretory states . 