user: Question: What is Date, when Meet is "World Championships", when Nationality is "United States", and when Time is "1:46.68"?
Columns: ['Event', 'Time', 'Nationality', 'Date', 'Meet', 'Location']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['50 m freestyle', '20.51', 'Brazil', '17 December 2010', 'World Championships', 'Dubai , United Arab Emirates']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Meet" = 'World Championships' AND "Nationality" = 'United States' AND "Time" = '1:46.68';