human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Gas name, Chemical formula, Lifetime (years), 20-yr, 100-yr, 500-yr
###Question: What is the 20 year for Sulfur Hexafluoride?
gpt
SELECT 20-yr FROM table WHERE Gas name = Sulfur hexafluoride