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: Multivariate analysis showed that trough TAC level was the only independent risk factor associated with the seizures .
### Output Text:
Multivariate analysis showed that trough <span class="chemical">TAC</span> level was the only independent risk factor associated with the seizures . 