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: Thus , we conclude that CE is a reasonably safe alternative to be used in subjects who do not tolerate P and N .
### Output Text:
Thus , we conclude that <span class="chemical">CE</span> is a reasonably safe alternative to be used in subjects who do not tolerate <span class="chemical">P</span> and <span class="chemical">N</span> . 