Database: retail_complains
Question: Give the client ID of the complaint received on April 16, 2014 and submitted through fax.
SQL: SELECT T2.Client_ID FROM callcenterlogs AS T1 INNER JOIN events AS T2 ON T1.`Complaint ID` = T2.`Complaint ID` WHERE T2.`Submitted via` = 'Fax' AND T1.`Date received` = '2014-04-16'