As property manager of a 15 -unit apartment block , you receive rent payments on the first of month from each tenant . Mrs . Lone , in apartment , whose rent is 5500 per month lost her job was in dire financial difficulties . You agreed to take only 5490 per month For the last ten mont she has paid only 5400 a month , but you have learned that for 4 months she has had a new even better- paying job . You are angry that she faced to tell you this and you want to sue he entire amount of back rent flood ) . Which of the following arguments could she use most effe against you ? promissory estoppel mistake undue influence ( breach of contract duress