Suzy bought groceries including some tinned food from Evergreen Supermarket for R800 on credit. On opening a tin of sardines a few days later she found them unfi t for human consumption and threw them away. When Evergreen Supermarket sent her an invoice of R800, she deducted R30, being the cost of the tin of sardines and sent in a cheque for R770 ‘in full settlement of their invoice’. She explained in the covering letter why she had deducted R30.Evergreen Supermarket banked her cheque and claimed the balance of R30 from Suzy, saying that they did not believe her story.Required:Advise Suzy. (10 marks)

氨水是有毒挥发刺激性气味的物质，使用时应戴（）。