human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Class, Railway number(s), Quantity, Year(s) of manufacture, Axle arrangement ( UIC )
###Question: What year was the b n2t axle arrangement, which has a quantity of 31, manufactured?
gpt
SELECT Year(s) of manufacture FROM table WHERE Quantity = 31 AND Axle arrangement ( UIC ) = b n2t