user: Question: Who is every vacator with reason for change as Resigned December 31, 1959?
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Missouri 4th', 'George H. Christopher (D)', 'Died January 23, 1959', 'William J. Randall (D)', 'March 3, 1959']
SQL:

assistant: SELECT "Vacator" FROM "Table" WHERE "Reason for change" = 'Resigned December 31, 1959';