user: Question: What is the production code of the episode written by Cathryn Humphris?
Columns: ['No.', '#', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (million)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [13.0, 1.0, '"Red Rain"', 'Bryan Spicer', 'Scott Rosenbaum & Gregg Hurwitz', 'January4,2011', '3X6201', '6.59']
SQL:

assistant: SELECT "Production code" FROM "Table" WHERE "Written by" = 'Cathryn Humphris';