Among all the active customers, how many of them live in Arlington?
active refers to active = 1; Arlington refers to city = 'Arlington'
SELECT COUNT(T2.customer_id) FROM address AS T1 INNER JOIN customer AS T2 ON T1.address_id = T2.address_id INNER JOIN city AS T3 ON T1.city_id = T3.city_id WHERE T2.active = 1 AND T3.city = 'Arlington'