What is the opening time of the active businesses in Glendale that have a medium review count.
active business ID refers to active = 'true'; Glendale is a city; medium review count refers to review_count = 'Medium'
SELECT DISTINCT T2.opening_time FROM Business AS T1 INNER JOIN Business_Hours AS T2 ON T1.business_id = T2.business_id INNER JOIN Days AS T3 ON T2.day_id = T3.day_id WHERE T1.city = 'Glendale' AND T1.review_count = 'Medium' AND T1.active = 'true'