What was the average price of a Bitcoin in the year 2013?
average price refers SUM(price)/COUNT(named = 'Bitcoin'); in the year 2013 refers to year(date) = 2013
SELECT AVG(T2.price) FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE STRFTIME('%Y', T2.date) = '2013' AND T1.name = 'Bitcoin'