Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is Date, when Meet is "World Championships", when Nationality is "United States", and when Time is "1:46.68"?

### Inputs:
table_name: table, columns: ['Event', 'Time', 'Nationality', 'Date', 'Meet', 'Location']

### Response:
SELECT Date FROM table WHERE Meet = world championships AND Nationality = united states AND Time = 1:46.68