Table: episode_info_7
Columns: no_in_series
no_in_season
title
directed_by
written_by
original_u_s_air_date
prod_code

Question: Who wrote episode 31 in the series?
SQL: SELECT "written_by" FROM episode_info_7 WHERE "no_in_series" = 31

Question: What was the date that the episode with a production code of 213 was originally aired?
SQL: SELECT "original_u_s_air_date" FROM episode_info_7 WHERE "prod_code" = 213