user: Question: How many ranks have a ationality of canada, and a Time of dns?
Columns: ['Rank', 'Heat : Lane', 'Name', 'Nationality', 'Time']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1.0, '5 : 4', 'Craig Stevens', 'Australia', '7:50.72']
SQL:

assistant: SELECT COUNT("Rank") FROM "Table" WHERE "Nationality" = 'canada' AND "Time" = 'dns';