user: Question: What year was the B n2t axle arrangement, which has a quantity of 31, manufactured?
Columns: ['Class', 'Railway number(s)', 'Quantity', 'Year(s) of manufacture', 'Axle arrangement ( UIC )']
Types: ['text', 'text', 'real', 'text', 'text']
Sample row: ['None', '70–73 ( de )', 4.0, '1868', 'B n2t']
SQL:

assistant: SELECT "Year(s) of manufacture" FROM "Table" WHERE "Quantity" = 31.0 AND "Axle arrangement ( UIC )" = 'B n2t';