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: We see the main application of NAA as for the combinational use in pharmaceutical preparations of acetaminophen in order to avoid hepatic damage in patients treated with this widely used analgesic .
### Output Text:
We see the main application of <span class="chemical">NAA</span> as for the combinational use in pharmaceutical preparations of <span class="chemical">acetaminophen</span> in order to avoid hepatic damage in patients treated with this widely used analgesic . 