How much is the average amount in credit card made by account holders in a month, in year 2021?
Operation = 'VYBER KARTOU' refers to credit card withdrawn
SELECT AVG(T3.amount) FROM card AS T1 INNER JOIN disp AS T2 ON T1.disp_id = T2.disp_id INNER JOIN trans AS T3 ON T2.account_id = T3.account_id WHERE STRFTIME('%Y', T3.date) = '2021' AND T3.operation = 'VYBER KARTOU'