human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Name, Year commissioned, Gross head (metres), Installed capacity (megawatts), Average annual output (million KWh)
###Question: What is the Year Commissioned of the power stationo with a Gross head of less than 18?
gpt
SELECT AVG Year commissioned FROM table WHERE Gross head (metres) < 18