user: Question: How many total viewers have April 21, 2010 as the original airing on channel 4?
Columns: ['Episode number', 'Title', 'Original airing on Channel 4', 'Time of airing on Channel 4', 'Original airing on E4', 'Time of airing on E4', 'Position in Channel 4s ratings a', 'Position in E4s ratings b', 'Total viewers']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [112.0, 'Nice is Different Than Good', 'January 27, 2010', '10.00pm-11.00pm', 'January 31, 2010', '10.00pm-11.00pm', '3.090m - #10', '328,000 - #11', '3.418m']
SQL:

assistant: SELECT "Total viewers" FROM "Table" WHERE "Original airing on Channel 4" = 'April 21, 2010';