user: Question: what is the regulatory citation when the standard is 15 μg/m³?
Columns: ['Pollutant', 'Type', 'Standard', 'Averaging Time', 'Regulatory Citation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['SO 2', 'Primary', '0.14 ppm (365 μg / m 3 )', '24-hour', '40 CFR 50.4(b)']
SQL:

assistant: SELECT "Regulatory Citation" FROM "Table" WHERE "Standard" = '15 μg/m³';