Which Illinois insurance line is subject to a file-and-use rate requirement rather than the open competition system?

Why this is the answer

While most Illinois P&C lines fall under the open competition system in 215 ILCS 5/Article XXIV ½ and require no prior approval, workers' compensation is treated differently. Under 215 ILCS 5/456, WC rates must be filed with the IDOI before use. NCCI develops loss cost filings on behalf of the industry, and each insurer files its loss cost multiplier with the Department.