请把答案以一个空格隔开，不需要标点符号 I am writing this letter to （投诉）the quality of the phone I bought last Tuesday in No. 1 Department store. The phone always （黑屏）. I called the _________(售后)， but the problem was not solved. Therefore, I have sent back the phone and the receipt to you and strongly insist that you ________________(退回我5000元) as soon as possible. I would be （感激的） if you could have this matter settled.