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 tested the hypothesis that nimodipine ( NIMO ) administered at the onset of nitroglycerin ( NTG ) - induced hypotension would preserve long - term associative memory .
### Output Text:
We tested the hypothesis that <span class="chemical">nimodipine</span> ( <span class="chemical">NIMO</span> ) administered at the onset of <span class="chemical">nitroglycerin</span> ( <span class="chemical">NTG</span> ) - induced hypotension would preserve long - term associative memory . 