user: Question: Which letter has an operator of the Defense Information Systems Agency?
Columns: ['Letter', 'IPv6 address', 'AS-number', 'Operator', 'Location #sites (global/local)', 'Software']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['A', '2001:503:ba3e::2:30', 'AS19836', 'Verisign', 'Distributed using anycast 8/0', 'BIND']
SQL:

assistant: SELECT "Letter" FROM "Table" WHERE "Operator" = 'Defense Information Systems Agency';