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: Early onset migraine attacks were induced by nitroglycerin in seven out of 10 patients versus no patient in the placebo group .
### Output Text:
Early onset migraine attacks were induced by <span class="chemical">nitroglycerin</span> in seven out of 10 patients versus no patient in the placebo group . 